Non-Linear Regression, aka Attributes Data Analysis

What is Non-Linear Regression?

Non-linear regression, aka Attributes Data Analysis, is organized into discrete data types (groups or categories).

When do you use Non-Linear Regression, aka Attributes Data Analysis?

Simple Linear regression is a statistical technique used to estimate the mathematical relationship between a dependent variable (usually denoted as Y) and an independent variable (usually denoted as X) and is denoted with a straight line y=mx+b. Although statistical linear models could describe the classic straight line, most statistically linear models are not represented by straight lines but by curvilinear graphs. Non-linear regression, aka Attributes Data Analysis, is used to explain the nonlinear relationship between a response variable and one or more predictor variables (mostly curve line). In other words, a regression model is called “non-linear” if the derivative of the model depends on one or more parameters.

Non- Linear Regression aka Attributes Data Analysis
Linear vs. nonlinear Regression

Specifically, use non-linear regression instead of ordinary least square regression when one cannot adequately model the relationship with linear parameters.

DO NOT USE Linear Regression for attribute data analysis- because this assumes the response variable is continuous.

For instance, Analyze attribute data using logit, probit, logistic regression, etc., to investigate sources of variation.

Logistic Regression

Logistic regression is a method used to predict a dependent variable for a given set of independent variables, such that the dependent variable is categorical. This method is widely used in machine learning algorithms.

Logistic regression is similar to linear regression; however, logistic regression predicts categorical data (like true or false), whereas linear regression predicts continuous data. Hence, the dependent variable in logistic regression is categorical, but in linear regression, the dependent variable is always continuous. Logistic regression uses a logit transformation on the dependent variable to fit a linear regression model.

The outcome of logistic regression is always categorical when the resultant outcome always has only two possible values of 0 or 1.  In logistic regression, the graph is not a linear line, but the line looks like a curve that goes between 0 and 1. The curve is called the “S” curve and is also called the Sigmoid curve.

Although logistic regression tells the probability, it is most commonly used for classification. The team has to determine the threshold value if the probability of the event happening between 0 and 1, then based on the threshold value event is to be classified as 0 or 1.

For example, if the threshold value is 0.5, then any value between 0.5 and 1 it should be classified as 1, similarly any value below 0.5 then it should classify as 0

Logistic Regression calculation

Non- Linear Regression aka Attributes Data Analysis

P ranges between 0 and 1; it represents the probability of an event to happen.

X1 and X2 are the independent variables that determine the occurrence of an event.

C is the constant

B1, B2 represents the respective coefficients of X1, X2

Additional Notes on Logistic Regression

  • The regression coefficients for logistic regression models are based on MAXIMUM LIKELIHOOD ESTIMATION.
  • Sample sizes should be at least 50 per variable.
  • Logistic Regression is best for:

Logistic Regression Example

We can use Non-linear regression models in the analysis phase of DMAIC. For example, the American Cancer Institute wants to predict the chances of throat cancer based on the smoker’s average cigarette consumption per day. From the below random data, 0 indicates no cancer, and 1 indicates throat cancer.

From the above data, logistic regression helps to assess what level of cigarette consumption leads to throat cancer. Moreover, from the above graph, one can say if the number of cigarette consumption is more than eight, it leads to throat cancer, and if cigarette consumption is below eight, there is no cancer. Hence, logistic regression is the best method for complex problems.

Certainly, we have to use statistical tools (like R and Minitab) to model the logistic regression.

Using the formula

Suppose we get the following values (random values) from the above information

C= -0.100245

B1= 0.1814

If we want to predict 2.6

=-0.100245+ 0.1814*2.6= -0.53081

Use the sigmoid function to find the probability

Since the f(z ) value is 0.37 (towards zero), we can predict 2.6, there will be no chance of throat cancer.

Logit Analysis

Logit Analysis

Logit analysis is used when the response variable is categorical, especially when the outcome has two possible results, such as pass/fail, yes/no, alive/dead, or defect/no defect.

The key idea behind logit analysis is that we model the log odds of an event, rather than modeling the probability directly.

The standard logit formula is:

logit(p) = ln(p / (1 – p))

Where:

  • p is the probability that the event occurs.
  • 1 – p is the probability that the event does not occur.
  • p / (1 – p) is the odds of the event occurring.
  • ln means the natural logarithm, or log base e.

This is an important notation point. In many statistics texts and software outputs, the word log often refers to the natural logarithm. However, to avoid confusion, it is clearer to write the formula as ln(p / (1 – p)).

Why Use the Logit?

Probabilities are limited to values between 0 and 1. That creates a problem if we try to model probability directly with a straight-line equation because a linear model can produce values below 0 or above 1.

The logit transformation solves this by converting probabilities into log odds. Log odds can range from negative infinity to positive infinity, which makes them suitable for regression modeling.

Once the regression model is built, the logit value can be converted back into a probability using the logistic function:

p = 1 / (1 + e-z)

Where:

  • p is the predicted probability of the event.
  • e is Euler’s number, approximately 2.718.
  • z is the linear prediction from the regression equation.

Logit Regression Equation

A basic logit regression model has the following form:

z = β0 + β1x1 + β2x2 + … + βnxn

Where:

  • β0 is the intercept.
  • β1, β2, … βn are the coefficients for the predictor variables.
  • x1, x2, … xn are the independent variables.

After calculating z, the logistic function converts that value into a probability between 0 and 1.

Logit Analysis Example

Suppose a cardiology doctor wants to predict the probability of a life-threatening outcome based on a patient’s cholesterol level. The response variable is binary: the patient is either alive or deceased.

The doctor may collect data showing cholesterol levels and the number of patients in each outcome group. From that data, the doctor can calculate the proportion of patients in the outcome category of interest.

The general steps are:

  1. Calculate the proportion: p = event / total.
  2. Calculate the odds: p / (1 – p).
  3. Calculate the logit using the natural logarithm: ln(p / (1 – p)).
  4. Use statistical software, such as Minitab, R, Python, or Excel, to estimate the regression equation.
  5. Use the final model to predict the probability of the event at a given cholesterol level.

For example, if the probability of an event is 0.75, then:

Odds = 0.75 / (1 – 0.75) = 0.75 / 0.25 = 3

Logit(p) = ln(3) = 1.099

This means the log odds of the event are 1.099. A positive logit value means the event is more likely than not. A negative logit value means the event is less likely than not. A logit value of 0 means the event has a probability of 0.5.

Common Source of Confusion: log vs. ln

One common source of confusion is the word log. In some settings, especially in basic math or engineering contexts, log may mean log base 10. In logistic regression, however, the logit function uses the natural logarithm.

For clarity, the formula should be written as:

logit(p) = ln(p / (1 – p))

Using log base 10 would produce different numerical values. The standard logit values used in logistic regression are based on the natural logarithm.

Once the model is created, the predicted logit can be converted back into a probability. This allows the team to estimate the likelihood of a specific outcome and classify the result if needed.

Probit Analysis

The probit model was first introduced by Chester Bliss in 1934, but Ronald Fisher proposed the maximum likelihood method as an appendix to Bliss in 1935.

A probit model is a popular ordinal or binary response model specification. This model, which employs a probit link function, was estimated using the maximum likelihood method, hence this estimation was named probit regression.

z= β0+ β1x1+ β2x2+………………+ βnxn

Where f is the standard normal cumulative distribution function (cdf)

Probit Analysis Example

A coaching center interested in students’ GMAT (Graduate Management Admission Test ) scores and admission into the business school. The response is a binary variable (i.e, admit or no admission into the business school)

Firstly, find the logarithm base 10 for the GMAT Score

Then, calculate the proportion of no admission = No admission /(admission + no admission)

Lastly, calculate the Probit values using the chart

Calculate the slope and intercept using probit values and log values of the GMAT score.

We must use statistical tools (like R and Minitab) to model the logistic regression.

Once the model was established, the probit function was to find whether the chances of student admission or no admission into the business school for a particular GMAT score.

Difference between logit and probit

Both logit and probit models yield almost the same results. The logit and probit functions are increasing, and both functions increase relatively quickly in the central portion and relatively slow at the extremities, and both functions lie between 0 and 1. While the only key difference between the logit and probit models is in the use of the link function

In other words, the logit model will have slightly flatter tails compared to the probit model. 

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ASQ Six Sigma Black Belt Study Guide

Preparing for the ASQ Six Sigma Black belt exam can be overwhelming. This ASQ Six Sigma Black Belt Study Guide lists all key concepts (and links to learn more) for the ASQ Six Sigma Black Belt Certification. This list comes from the 2022 ASQ Black Belt BOK update. And it’s totally free to use.

What is the ASQ Six Sigma Black Belt BOK?

The ASQ Six Sigma Black Belt BOK was created by the American Society of Quality (ASQ). It’s periodically reviewed and updated by leading members and Six Sigma professionals to ensure it’s relevant to today’s world.

The ASQ Six Sigma Black Belt BOK has nine sections geared around Six Sigma Fundamentals, Organizational Deployment, the different phases of DMAIC, and Design for Six Sigma (DFSS).

You’ll need to master various topics to pass the exam.

How to use the ASQ Six Sigma Black Belt Study Guide

Some ASQ Six Sigma Black Belt candidates will have taken Six Sigma training. Others will be self-taught. No matter your path, you must know what’s on the standard BOK – Body of Knowledge.

Step 1: Scan over the material below.

Scan through the rest of this page before proceeding further. Take a look at what will be asked. Do you feel comfortable with this material? Is anything new to you? Are there any surprises?

Make a mental note of that and proceed.

Step 2: Review (or learn!) the material.

Not every training program covers every topic. Depending on the quality of the training you took, your instructor may have skipped a section entirely! Click one of the links to dive into any topic that you don’t feel as strongly as you’d like and read up.

Step 3: Leave a comment on any page where you have questions.

Each topic below is linked to a detailed article where I thoroughly review the concept. If you have any questions, or if any topic is less clear than you’d like it to be, just leave a comment, and I’ll get back to you ASAP.

Step 4: Practice as many questions as possible.

Find an excellent question bank covering each section, and take as many practice tests as possible.

The ASQ Six Sigma Black Belt BOK

What follows below is the ASQ Six Sigma Black Belt BOK. The top-level sections I through IX also list the number of questions you can expect in that section. This will help you see the relative weight of those topics compared to others.

Use this page as your ASQ Six Sigma Black Belt Study Guide. All Body of Knowledge topics required for the exam are mapped to study notes and example problems. Also, read here to see how I prepared and passed the ASQ Black Belt exam.

I. Organization-Wide Planning and Deployment (12 Questions)

I.A. Organization-wide Considerations

I.A.1. Fundamentals of Six Sigma and Lean Methodologies

Define and describe the value and goals of these approaches (Six Sigma, Lean), and describe the integration and complementary relationship between them. Identify and understand an organization’s lean Six Sigma maturity model. (Understand)

I.A.2. Six Sigma, lean, and continuous improvement methodologies

Describe when to use Six Sigma instead of other problem-solving approaches, and describe the importance of aligning Six Sigma objectives with organizational goals. Describe screening criteria and how such criteria can be used for the selection of Six Sigma projects, lean initiatives, and other continuous improvement methods. (Apply) (Also see the History of Six Sigma and the History of Lean. )

I.A.3. Relationships among business systems and processes

Describe the interactive relationships among business systems, processes, and internal and external stakeholders, and the impact those relationships have on business systems. (Understand)

I.A.4. Strategic planning and deployment for initiatives

Define the importance of identification and strategic planning of Six Sigma projects and lean initiatives as part of the business planning process taking into consideration outcomes such as return on investment and measured intangibles. Use strengths, weaknesses, opportunities, and threats analysis (SWOT), contingency planning, and business continuity planning to enhance strategic planning and deployment. (Apply)

I.B. Leadership

I.B.1. Roles and responsibilities

Describe the roles and responsibilities of executive leadership, champions, sponsors, process owners, and Master Black Belts, Black Belts, and Green Belts in driving Six Sigma and lean initiatives. Understand the importance of coaching multiple levels of leadership and belts on appropriate six sigma tools and techniques. Understand the importance of finance and its role in supporting a project and confirming its outcome. Describe how each group influences project deployment in terms of providing or managing resources, enabling changes in organizational structure, and supporting communications about the purpose and deployment of the initiatives. (Understand)

I.B.2. Organizational barriers

Describe how an organization’s structure and culture can impact Six Sigma projects. Identify common causes of Six Sigma failures, including lack of management support and lack of resources. (Apply) Also, see Critical success factors.

I.B.3. Change Management

Apply and facilitate change management techniques, including stakeholder analysis, readiness assessments, proactive change management, and communication plans to overcome barriers and drive organization-wide change. (Apply)

II. Organizational Process Management and Measures (12 Questions)

II.A. Impact on Stakeholders

Describe the impact Six Sigma projects can have on customers, suppliers, and other stakeholders. (Understand)

II.B. Benchmarking

Define and distinguish between various types of benchmarking, e.g., best practices, competitive, collaborative, and breakthrough. Select measures and performance goals for projects resulting from benchmarking activities. (Apply)

II.C. Business Measures

II.C.1. Performance measures

Define and describe a balanced scorecard, key performance indicators (KPIs), key behavior indicators (KBIs), objectives and key results (OKRs), customer loyalty metrics, and leading and lagging indicators. Explain how to create a line of sight from performance measures to organizational strategies. (Analyze)

II.C.2. Financial measures

Define and use revenue growth, market share, margin, net present value (NPV), return on investment (ROI), and cost-benefit analysis (CBA). Explain the difference between hard cost measures (from profit and loss statements) and soft cost benefits of cost avoidance and reduction. (Apply)

III. Team Management (15 Questions)

III.A. Team Formation

III.A.1. Team types and constraints

Define and describe various teams, including virtual, cross-functional, and self-directed. Determine what team type will work best for a given set of constraints, e.g., geography, technology availability, staff schedules, and time zones. (Apply)

III.A.2. Team roles and responsibilities

Define and describe various team roles and responsibilities for the leader, facilitator, coach, and individual member. (Understand)

III.A.3. Team member selection criteria

Describe various factors that influence the selection of team members, including the ability to influence, openness to change, required skill sets, subject matter expertise, and availability. (Apply)

III.A.4. Team success factors

Identify and describe the elements necessary for successful teams, e.g., management support, clear goals, ground rules, and timelines. (Apply)

III.B. Team Facilitation

III.B.1. Motivational techniques

Describe and apply techniques to motivate team members. Identify factors that can demotivate team members and describe techniques to overcome them. (Apply)

III.B.2. Team stages of development

Identify and describe the classic stages of team development: forming, storming, norming, performing, and adjourning. (Apply)

III.B.3. Team communication

Describe and explain the elements of an effective communication plan, e.g., audience identification, message type, medium, and frequency. (Apply)

III.B.4. Team leadership models

Describe and select appropriate leadership approaches (e.g., direct, coach, support, and
delegate) to ensure team success. (Apply)

III.C. Team Dynamics

III.C.1. Group behaviors

Identify and use various conflict resolution techniques (e.g., coaching, mentoring, and intervention) to overcome negative group dynamics, including dominant and reluctant participants, groupthink, rushing to finish, and digressions. (Evaluate)

III.C.2. Meeting management

Select and use various meeting management techniques, including using agendas, starting on time, requiring pre-work by attendees, and ensuring that the right people and resources are available. (Apply)

III.C.3. Team decision-making methods

Define, select, and use various tools (e.g., consensus, nominal group technique, and multi-voting) for decision-making. (Apply)

III.D. Team Training

III.D.1. Needs assessment

Identify the steps involved to implement an effective training curriculum: identify skills gaps, develop learning objectives, prepare a training plan, and develop training materials. (Understand)

III.D.2. Delivery

Describe various techniques used to deliver effective training, including adult learning theory, soft skills, and modes of learning. (Understand)

III.D.3. Evaluation

Describe various techniques to evaluate training, including evaluation planning, feedback surveys, pre-training, and post-training testing. (Understand)

IV. Define (20 Questions)

IV.A. Voice of the Customer

IV.A.1. Customer Identification

Identify and segment customers and show how a project will impact both internal and external customers. (Apply)

IV.A.2. Customer data collection

Identify and select appropriate data collection methods (e.g., surveys, focus groups, interviews, and observations) to gather the voice of the customer data. Ensure the data collection methods used are reviewed for validity and reliability. (Analyze)

IV.A.3. Customer requirements

Define, select, and apply appropriate tools to determine customer needs and requirements, including critical-to-X (CTX when “X” can be quality, cost, safety, etc.), CTQ tree, quality function deployment (QFD), supplier, input, process, output, customer (SIPOC), and Kano model. (Analyze)

IV.B. Business Case and Project Charter

IV.B.1. Business case

Describe the business case justification used to support projects. (Understand)

IV.B.2. Problem statement

Develop a project problem statement and evaluate it in relation to baseline performance and improvement goals. (Evaluate)

IV.B.3. Project scope

Develop and review project boundaries to ensure that the project has value to the customer. (Analyze)

IV.B.4. Goals and objectives

Identify SMART (specific, measurable, actionable, relevant, and timebound) goals and objectives on the basis of the project’s problem statement and scope. (Analyze)

IV.B.5. Project performance measurements

Identify and evaluate performance measurements (e.g., cost, revenue, delivery, schedule, and customer satisfaction) that connect critical elements of the process to key outputs. (Analyze)

IV.B.6. Project charter review

Explain the importance of having periodic project charter reviews with stakeholders. (Understand)

IV.C. Project Management (PM) Tools

Identify and use the following PM tools to track projects and document their progress. (Evaluate)

IV.C.1. Gantt charts

IV.C.2. Toll-gate reviews

IV.C.3. Work breakdown structure (WBS)

IV.C.4. RACI model (responsible, accountable, consulted, and informed)

IV.D. Analytical Tools

Identify and use the following analytical tools throughout the DMAIC cycle. (Apply)

IV.D.1. Affinity diagrams

IV.D.2. Tree diagrams

IV.D.3. Matrix diagrams

IV.D.4. Prioritization matrices

IV.D.5. Activity network diagrams

IV.D.6. Process decision program chart (PDPC)

IV.D.7. Interrelationship digraph (ID)

V. Measure (25 Questions)

V.A. Process Characteristics

V.A.1. Process flow metrics

Identify and use process flow metrics (e.g., work in progress (WIP), work in queue (WIQ), touch time, takt time, cycle time, and throughput) to determine constraints. Describe the impact that “hidden factories” can have on process flow metrics. (Analyze)

V.A.2. Process analysis tools

Select, use, and evaluate various tools, e.g., value stream maps, process maps, work instructions, flowcharts, spaghetti diagrams, and Gemba walk. (Evaluate)

V.B. Data Collection

V.B.1. Types of data

Define, classify, and distinguish between qualitative and quantitative data and continuous and discrete data. (Evaluate)

V.B.2. Measurement scales

Define and use nominal, ordinal, interval, and ratio measurement scales. (Apply)

V.B.3. Sampling

Define and describe sampling concepts, including representative selection, homogeneity, bias, accuracy, and precision. Determine the appropriate sampling method (e.g., random, stratified, systematic, subgroup, and block) to obtain valid representation in various situations. (Evaluate) Also, see Sampling methods.

V.B.4. Data collection plans and methods

Develop and implement data collection plans that include data integrity, accuracy, and
processing tools, e.g., check sheets, and data normalization. Avoid data collection pitfalls by defining the metrics to be used or collected, ensuring that collectors are trained in the tools and understand how the data will be used, and checking for seasonality effects. (Analyze)

V.C. Measurement Systems

V.C.1. Measurement system analysis (MSA)

Use gauge repeatability and reproducibility (R&R) studies and other MSA tools (e.g., bias, correlation, linearity, precision to tolerance, and percent agreement) to analyze variable measurement system capability. Use audit MSA for an attribute measurement system. (Evaluate)

V.C.2. Measurement systems across the organization

Identify how measurement systems can be applied across all functional areas of the organization (e.g., marketing, sales, engineering, research and development (R&D), supply chain management, operations, and customer experience). (Understand)

V.C.3. Metrology

Define and describe elements of metrology, including calibration systems, traceability to reference standards, and the control and integrity of measurement devices and standards. (Understand) Also, see Evaluating the Measurement Systems.

V.D. Basic Statistics

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V.D.1. Basic statistical terms

Define and distinguish between population parameters and sample statistics, e.g., proportion, mean, and standard deviation. (Apply)

V.D.2. Central limit theorem

Explain the central limit theorem and its significance in the application of inferential statistics for confidence intervals, hypothesis tests, and control charts. (Understand)

V.D.3. Descriptive statistics

Calculate and interpret measures of dispersion and central tendency. (Evaluate)

V.D.4. Graphical methods

Construct and interpret diagrams and charts, e.g., box-and-whisker plots, scatter diagrams, histograms, normal probability plots, frequency distributions, and cumulative frequency distributions. (Evaluate) (Also see Frequency Plots)

V.D.5. Valid statistical conclusions

Distinguish between descriptive and inferential statistical studies. Evaluate how the results of statistical studies are used to draw valid conclusions — see the key differences between the two and what each is used to conclude. (Evaluate)

V.E. Probability

V.E.1. Basic concepts

Describe and interpret probability concepts, e.g., independence, mutually exclusive events, addition and multiplication rules, conditional probability, complementary probability, and joint occurrence of events. (Understand)

V.E.2. Common distributions

Describe, interpret, and use normal, Poisson, binomial, chi-square, Student’s t, and F distributions. (Evaluate)

V.E.3. Additional distributions

Identify hypergeometric, bivariate, exponential, lognormal, and Weibull distributions. (Understand)

V.F. Process Capability

V.F.1. Process capability indices

Define, select, and calculate Cp and Cpk. (Evaluate)

V.F.2. Process performance indices

Define, select, and calculate Pp, Ppk, Cpm, and process sigma. (Evaluate)

V.F.3. General process capability studies

Describe and apply elements of designing and conducting process capability studies relative to characteristics, specifications, sampling plans, stability, and normality. (Evaluate)

V.F.4. Process capability for attributes data

Calculate the process capability and process sigma level for attributes data. (Apply)

V.F.5. Process capability for non-normal data

Identify non-normal data and determine when it is appropriate to use Box-Cox or other transformation techniques. (Apply)

V.F.6. Process performance vs. specification

Distinguish between natural process limits and specification limits. Calculate process
performance metrics
, e.g., percent defective, parts per million (PPM), defects per million opportunities (DPMO), defects per unit (DPU), first pass yield, and rolled throughput yield (RTY). (Evaluate)

V.F.7. Short-term and long-term capability

Describe and use appropriate assumptions and conventions when only short-term data or only long-term data are available. Interpret the relationship between short-term and long-term capability. (Evaluate)

VI. Analyze (22 Questions)

VI.A. Measuring and Modeling Relationships Between Variables

VI.A.1. Correlation coefficient

Calculate and interpret the correlation coefficient and its confidence interval and describe
the difference between correlation and causation. (Evaluate)

VI.A.2. Linear regression

Calculate and interpret regression analysis and apply and interpret hypothesis tests
for regression statistics. Use the regression model for estimation and prediction, analyze the
uncertainty in the estimate, and perform a residuals analysis to validate the model. (Evaluate)

VI.A.3. Multivariate tools

Understand sources of variation through multivariate tools (e.g., factor analysis, discriminant
analysis, and multiple analysis of variance (MANOVA). (Understand)

VI.B. Hypothesis Testing

VI.B.1. Terminology

Define and interpret the significance level, power, type I, and type II errors of statistical tests. (Evaluate)

VI.B.2. Statistical vs. practical significance

Define, compare, and interpret the statistical and practical significance. (Evaluate)

VI.B.3. Sample size

Calculate sample size for common hypothesis tests: equality of means and equality of proportions. (Apply)

VI.B.4. Point and interval estimates

Define and distinguish between confidence and prediction intervals. Define and interpret the efficiency and bias of estimators. Calculate tolerance and confidence intervals. (Evaluate)

VI.B.5. Tests for means, variances, and proportions

Use and interpret the results of hypothesis tests for means, variances, and proportions. (Evaluate)

VI.B.6. Analysis of variance (ANOVA)

Select, calculate, and interpret the results of ANOVAs. (Evaluate)

VI.B.7. Goodness-of-fit (chi-square) tests

Define, select, and interpret the results of these tests. (Evaluate)

VI.C. Risk Analysis and Management

VI.C.1. Types of risk

Identify, assess, and prioritize various types of risk such as enterprise, operational, supplier, security, product, and cyber-security. (Analyze)

VI.C.2. Failure mode and effects analysis

Describe, the purpose and elements of FMEA, including risk priority number (RPN), and evaluate FMEA results for processes, products, and services. Distinguish between design FMEA (DFMEA) and process FMEA (PFMEA), and interpret their results. (Evaluate)

VI.D. Additional Analysis Methods

VI.D.1. Gap analysis

Analyze scenarios to identify performance gaps and compare current and future states using
predefined metrics. (Analyze)

VI.D.2. Root cause analysis

Define and describe the purpose of root cause analysis, recognize the issues involved in identifying a root cause, and use various tools (e.g., 5 whys, Pareto charts, fault tree analysis, cause and effect diagrams, and A3) to resolve chronic problems. (Analyze)

VI.D.3. Waste analysis

Identify and interpret the seven classic wastes (overproduction, inventory, defects, over-processing, waiting, motion, and transportation) and resource under-utilization. (Analyze)

VII. Improve (21 Questions)

VII.A. Design of Experiments (DOE)

VII.A.1. Terminology

Define basic DOE terms, e.g., independent and dependent variables, factors and levels, response, treatment, error, and nested. (Understand)

VII.A.2. Design principles

Define and apply DOE principles, e.g., power, sample size, balance, repetition, replication,
order, efficiency, randomization, blocking, interaction, confounding, and resolution. (Apply)

VII.A.3. Planning experiments

Plan and evaluate DOEs by determining the objective, selecting appropriate factors, responses,
and measurement methods, and choosing the appropriate design. (Evaluate)

VII.A.4. One-factor experiments

Understand when to use completely randomized, randomized block, and Latin square designs. (Understand)

VII.A.5. Two-level fractional factorial experiments

Understand these types of experiments and describe how confounding can affect their use. (Understand)

VII.A.6. Full factorial experiments

Understand these types of experiments. (Understand)

VII.B. Lean Methods

VII.B.1. Waste elimination

Select and apply tools and techniques for eliminating or preventing waste, e.g., pull systems, kanban, 5S, standard work, and poka-yoke. (Analyze)

VII.B.2. Cycle-time reduction

Use various tools and techniques for reducing cycle time, e.g., continuous flow, single-minute exchange of die (SMED), and heijunka (production leveling). (Analyze)

VII.B.3. Kaizen

Define and distinguish between kaizen and kaizen blitz and describe when to use each method. (Apply)

VII.B.4. Other improvement tools and techniques

Identify and describe how other process improvement methodologies are used, e.g., theory of constraints (TOC) and overall equipment effectiveness (OEE). (Understand)

VII.C. Implementation

Develop plans for implementing proposed improvements, including conducting pilot tests or simulations, and evaluate results to select the optimum solution. (Evaluate)

VIII. Control (17 Questions)

VIII.A. Statistical Process Control (SPC)

VIII.A.1. Objectives

Explain the objectives of SPC, including monitoring and controlling process performance, tracking trends and runs, and reducing variation within a process. (Understand)

VIII.A.2. Selection of variables

Identify and select critical process characteristics for control chart monitoring. (Apply)

VIII.A.3. Rational subgrouping

Define and apply the principle of rational subgrouping. (Apply)

VIII.A.4. Control chart selection

Select and use control charts in various situations: X-R, X-s, individual and moving range (ImR), p, np, c, u, short-run SPC, and moving average. (Apply)

VIII.A.5. Control chart analysis

Interpret control charts and distinguish between common and special causes using rules for determining statistical control. (Analyze)

VIII.B. Other Controls

VIII.B.1. Total productive maintenance (TPM)

Define the elements of TPM and describe how it can be used to consistently control the improved process. (Understand)

VIII.B.2. Visual controls

Define the elements of visual controls (e.g., pictures of correct procedures, color-coded components, and indicator lights), and describe how they can help control the improved process. (Understand)

VIII.C. Maintain Controls

VIII.C.1. Measurement system reanalysis

Review and evaluate measurement system capability as process capability improves and ensure that measurement capability is sufficient for its intended use. (Evaluate)

VIII.C.2. Control plan

Develop a control plan to maintain the improved process performance, enable continuous improvement, and transfer responsibility from the project team to the process owner. (Apply)

VIII.D. Sustain Improvements

VIII.D.1. Lessons learned

Document the lessons learned and benefits realized from all phases of a project and identify strategies for reinforcing and replicating improvements. (Apply)

VIII.D.2. Documentation

Develop or modify documents, including standard operating procedures (SOPs), work instructions, and control plans, to ensure that the improvements are sustained over time. (Apply)

VIII.D.3. Training for process owners and staff

Develop and implement training plans that are handed off to process owners to ensure consistent execution of revised process methods, KPIs to confirm sustained benefits, and standards to maintain process improvements. (Apply)

VIII.D.4. Ongoing evaluation

Identify and apply tools (e.g., control charts and control plans) for ongoing evaluation of the
improved process, including monitoring leading indicators, lagging indicators, and
additional opportunities for improvement. (Apply)

IX. Design for Six Sigma (DFSS) Framework and Methodologies (6 Questions)

IX.A. Common DFSS Methodologies

Identify, describe, define, measure, analyze, design, and validate (DMADV) and define, measure, analyze, design, optimize, and validate (DMADOV). (Understand)

IX.B. Design for X (DFX)

Describe design constraints, including design for cost, design for manufacturability (producibility), design for test, and design for maintainability. (Understand)

IX.C. Robust Designs

Describe the elements of robust product design, tolerance design, and statistical tolerancing. (Understand)

ASQ Six Sigma Black Belt BOK Helpful Resources

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You’ve spent so much effort learning Lean Six Sigma. Why leave passing your certification exam up to chance? This comprehensive study guide offers 1,000+ exam-like questions for Green Belts (2,000+ for Black Belts) with full answer walkthroughs, access to instructors, detailed study material, and more.

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Villanova Six Sigma Black Belt Study Guide

The Villanova Six Sigma Black Belt Study Guide is a free, quick-reference list of essential materials to prepare for and pass the certification exam. Master the Villanova Six Sigma Blackbelt Body of Knowledge with this Study Guide.

Speaking of passing their exam, did you know that my Black Belt study guide comes with a pass-or-refund guarantee? Complete the course, and if you don’t pass your exam (IASSC, ASQ, Villanova, etc), I’ll refund you. Sign up here.

Villanova Six Sigma Black Belt Study Guide

The Villanova Six Sigma Blackbelt exam is structured around their suggested body of knowledge. I took and passed this test in May 2014 by following the structure below. In order to make the best use of my time I organized my study by following the outline below. For each subject, I translated my paper notes and many internet bookmarks and turned to this website.

Each link goes to a page of my notes. There are many other pages on this website for six sigma-related subjects not linked here but I felt were useful. This was especially helpful during exam time as Villanova states that their exam is far more intensive than their offered study materials. In other words, they expect you to master the industry and you will be tested on far more than what they teach you. In fact, the head of Six Sigma education at Villanova sent a notice to all students stating the exam is comprehensive of the industry and far more intensive than what would be a final exam (see comment here from Dan Munson).

I. Importance of Six Sigma

a. Value of Six Sigma

i. Six Sigma-basic premise

Describe how Six Sigma has fundamentally two focuses: variation reduction and waste reduction that ultimately lead to fewer defects and increased efficiency.

Understand the concept of variation and how the six Ms have an influence on the process. Understand the difference between assignable-cause and common cause variation along with how to deal with each type.

ii. History of organizational improvement/foundations of Six Sigma

Understand the origin of continuous improvement tools used in Six Sigma (e.g. Deming, Juran, Shewhart, Ishikawa, Taguchi, etc.)

iii. Quality improvement concepts

Define and describe concepts such as prevention versus detection, quality characteristics (i.e. nominal-is-best, smaller-is-best, or larger-is-best), and how they apply to target values.

iv. Value to the enterprise

Recognize why organizations use Six Sigma, and how they apply its philosophy and goals.

Describe how process inputs, outputs, and feedback impact the larger organization. Critical Success Factors to Six Sigma

v. Drivers and metrics

Recognize key drivers for business [profit, market share, customer satisfaction, efficiency, product differentiation, defects per million opportunities (DPMO), etc.] and how key metrics are developed and impact the entire organization.

vi. Financial Benefits

Understand and present financial measures and other benefits (soft and hard) of a project; understand and use basic financial models (e.g., NPV, ROI);

vii. Cost of Poor Quality

Describe, apply, evaluate, and interpret the cost of quality concepts, including quality cost categories, data collection, reporting, etc.

viii. Business system and processes

Understand and distinguish interrelationships between business systems and processes.

b. Voice of the customer

i. Customer focus

Identify and classify internal and external customers as applicable to a particular project, and show how projects impact customers.

ii. Collect customer data

Use various methods to collect customer feedback (e.g. Kano diagrams, observation, etc.) and identify key elements that make these tools effective.

iii. Analyze customer data

Use graphical, statistical, and qualitative tools to analyze customer feedback.

iv. Translate customer requirements

Assist in translating customer feedback into project goals and objectives, including critical to quality (CTQ) attributes and requirements statements. Use the voice of customer analysis tools such as quality function deployment (QED) and Kano diagrams to translate customer requirements into performance measures.

v. Owners and stakeholders

Identify process owners, internal and external customers, and other stakeholders in a project.

II. Six Sigma and the organization

a. Six Sigma projects

i. Project charter and problem statement

Define and describe elements of a project charter and develop a problem statement, including baseline and improvement goals.

ii. Charter negotiation

Create and negotiate the charter, including objectives, scope, boundaries, project transition, and project closure.

iii. Problem statement

Develop a problem statement, including baseline and improvement goals.

iv. Metrics

Establish primary and consequential metrics (quality, cycle time, cost)

v. Process elements

Define and describe process components and boundaries.

vi. Processes

Recognize how processes across various functional areas and the challenges that result from process improvement efforts.

vii. Project planning tools

Use project tools such as critical path method (CPM), program evaluation and review technique (PERT) charts, etc.

viii. Project documentation

Create data-driven and fact-driven project documentation using spreadsheets, storyboards, phased reviews, management reviews, presentations to the executive team, etc.

ix. DMAIC

Describe the phases of the DMAIC model including tollgate reviews.

x. Project metrics

Assist with the development of primary and consequential metrics (e.g., quality time, cycle time, and cost) and establish key project metrics that relate to the voice of the customer.

xi. Project risk analysis

Describe the purpose and benefit of project risk analysis, including resources, financials, impact on customers and other stakeholders, etc.

xii. Project closure

Describe the objectives achieved and apply the lessons learned to identify additional opportunities.

b. Teams

i. Initiating teams

Know the elements of launching a team and why they are important: clear purpose, goals, commitment, ground rules, roles and responsibilities of team members, schedules, support from management, and team empowerment.

ii. Selecting team members

Select team members that have appropriate skillsets (e.g., self-facilitation, technical/subject matter expertise), and create teams with appropriate numbers of members and representation.

iii. Team stages and dynamics

Define and describe the stages of team evolution, including forming storming, norming, and performing. Identify and help resolve negative dynamics such as overbearing, dominant, or reluctant participants, the unquestioned acceptance of opinions and facts, groupthink, feuding, floundering, the rush to accomplishment, etc.

iv. Roles and responsibilities

Describe and define the roles and responsibilities of participants on Six Sigma and other teams, including Black Belt, Master Black Belt, Green Belt, champion, coach, facilitator, team member, sponsor, process owner, etc.

v. Team tools

Define, select, and apply team tools such as nominal group technique, multivoting, etc.

vi. Team facilitation techniques

Apply coaching, mentoring, and facilitation techniques to guide a team and overcome problems such as overbearing, dominant, or reluctant participants, the unquestioned acceptance of opinions as facts, tangents, etc.

vii. Team-building techniques

Recognize and apply the basic steps in team building: goals, roles and responsibilities, introductions and both stated and hidden agendas.

viii. Team performance evaluation

Measure team progress in relation to goals, objectives, and metrics that support team success.

ix. Motivation techniques

Define, select, and apply techniques that support and sustain team member participation and commitment.

x. Communication

Use effective and appropriate communication techniques for different situations to overcome barriers to project success.

xi. Negotiation and conflict resolution techniques

Define, select, and apply tools such as consensus techniques, brainstorming, effort/impact, multivoting, and interest-based bargaining to help conflicting parties (e.g., departments, groups, leaders, staff) recognize common goals and how to work together to achieve them.

c. Change Management

i. Managing change

Understand and apply techniques for facilitating or managing organizational change through change agent methodologies.

ii. Organizational roadblocks

Understand the inherent structures of an organization (e.g., its cultures and constructs) that present basic barriers to improvement; select and apply techniques to overcome them.

d. Senior management elements

i. Organizational leadership

Understanding the leadership roles in the deployment of Six Sigma.

ii. Benchmarking

Understand the importance of benchmarking.

iii. Strategic planning

Describe the key elements of strategic business planning and describe why it is important at the project level and at the top level of an organization.

iv. Supply chain management

Describe why the development of suppliers is important to a Six Sigma initiative.

v. Barriers to implementation

Describe how to handle some of the common barriers to implementation.

III. Six Sigma objectives

a. Key process and product/service parameters identified

Identify key process input variables and process output variables (SIPOC), document their relationships through relational matrices, etc.

Develop and review process maps, flowcharts, failure modes and effects analysis (FMEA), etc.

b. Measurement systems that are discriminate, repeatable, and reproducible.

Calculate, analyze, and interpret measurement system capability using control chart methods.

c. Processes in statistical control

Identify, select, construct, and apply the following types of control charts: X-bar and R, XmR, p, np, c, and u.

Describe how turning attribute data into variable discrete data.

Understand how to make ‘active use’ of attribute data.

d. Statistically capable processes

Identify, describe, and apply the elements of designing and conducting process capability studies, including identifying characteristics, identifying specifications, and verifying stability and normality.

Define, select, and calculate Cp and Cpk, and assess process capability.

e. Long-term capability

Describe the assumptions and conventions that are appropriate when only short-term data are collected and when only attribute data are available.

Describe the changes in relationships that occur when long-term data are used, and interpret the relationship between long- and short-term capability as it relates to a 1.5 sigma shift.

f. Visual workplace/factory

Identify the key aspects of a visual workplace

g. Standard work

Identify the key aspects of standard work.

IV. Six Sigma Approach

a. Process analysis and documentation

i. Quality Function Deployment (QFD)

Describe how the House of Quality fits into the overall DFSS process.

ii. Process mapping

Develop and review process maps, written procedures, work instructions, flowcharts, etc.

iii. Failure mode and effects analysis (FMEA)

Define and describe failure mode and effects analysis (FMEA).

Describe the purpose and use of scale criteria and calculate the risk priority number (RPN).

iv. Data collection methods

Identify and classify continuous (variables) and discrete (attributes) data.

v. Techniques for assuring data accuracy and integrity

Define and apply techniques such as random sampling, sample homogeneity, etc.

vi. Multi-vari studies

Create and interpret multi-vari studies to interpret the difference between within-piece, piece-to-piece, and time-to-time variation.

b. Probability and statistics

Rusty on the math behind these topics? Join the Math Confidence for Six Sigma waitlist and I’ll let you know when the course opens.

i. Basic probability concepts

Describe and apply concepts such as independence, mutually exclusive, multiplication rules, complementary probability, the joint occurrence of events, etc.

ii. Descriptive statistics

Define, compute, and interpret measures of dispersion and central tendency, and construct and interpret frequency distributions and cumulative frequency distributions.

iii. Graphical methods

Depict relationships by constructing, applying, and interpreting diagrams and charts such as stem-and-leaf plots, box-and-whisker plots, run charts, scatter diagrams, etc., and depict distributions by constructing, applying, and interpreting diagrams such as histograms, normal probability plots, etc.

c. Collecting and summarizing data

i. Types of data

Identify, define, classify, and compare continuous (variables) and

ii. discrete (attributes) data, and recognize opportunities to convert attribute data to variable measures.

iii. Measurement scales

Define and apply nominal, ordinal, interval, and ratio measurement scales.

iv. Methods for collecting data

Define and apply methods for collecting data such as coding data, automatic gaging, etc.

v. Techniques for assuring data accuracy and integrity

Define and apply techniques for assuring data accuracy and integrity such as random sampling, stratified sampling, sample homogeneity, etc.

d. Hypothesis testing

i. Fundamental concepts of hypothesis testing

1. Statistical vs. practical significance

Define, compare, and contrast statistical and practical significance.

2. Significance level, power, type I and type II errors

Apply and interpret the significance level, power, and type I and type II errors of statistical tests.

3. Sample size

Understand how to calculate sample size for any given hypothesis test.

ii. Point and interval estimation

Define and interpret the efficiency and bias of estimators; compute, interpret, and draw conclusions from statistics such as standard error, tolerance intervals, and confidence intervals; understand the distinction between confidence intervals and prediction intervals.

iii. Test for means, variances, and proportions

Apply hypothesis tests for means, variances, and proportions, and interpret the results.

iv. Paired-comparison tests

Define, determine applicability, and apply paired-comparison parametric hypothesis tests and interpret the results.

v. Goodness-of-fit tests

Define, determine applicability, apply chi-square tests, and interpret the results.

vi. Analysis of Variance (ANOVA)

Define, determine applicability, apply ANOVAs, and interpret the results.

vii. Contingency Tables

Define, determine applicability, and construct a contingency table and use it to determine statistical significance.

e. Properties and applications of probability distributions

i. Distributions commonly used by black belts

Describe and apply binomial, Poisson, normal, chi-square, Student’s t, and F distributions.

ii. Other distributions

Recognize when to use hypergeometric, bivariate, exponential, and lognormal distributions.

f. Measurement systems

i. Measurement methods

Describe and review measurement methods such as attribute screens, gauge blocks, calipers, micrometers, optical comparators, tensile strength, titration, etc.

ii. Measurement system analysis

Calculate, analyze, and interpret measurement system capability using repeatability and reproducibility, measurement correlation, bias, linearity, percent agreement, precision/tolerance (P/T), precision/total variation (P/TV), and use both ANOVA and control chart methods for non-destructive, destructive, and attribute systems.

iii. Metrology

Understand traceability to calibration standards, measurement error, calibration systems, control, and integrity of standards and measurement devices.

g. Lean enterprise

i. Value-added and non-value-added activities

Identify waste in terms of excess inventory, space, test inspection, rework transportation, storage, etc., and reduce cycle time to improve throughput.

ii. Characterized the classic wastes

Define and be able to differentiate the wastes (muda). (i.e., defects, overproduction, waiting, non-utilized talent, transportation, inventory, motion, and extra processing).

iii. Value stream mapping

Define and apply methods for construction including calculating takt time, lead time, cycle time, etc.

iv. Quick change-overs

Describe the basic concepts regarding a quick change-over and why it is important to a lean enterprise.

V. Quality improvement tools and techniques

a. Tools most commonly used in projects

i. Ishikawa tools

Describe the objectives and benefits of the cause-effect diagram, check sheet, Pareto chart, graphs, control charts, scatter diagram and histogram.

ii. New management tools

Define, select, and use

1) affinity diagrams,

2) interrelationship diagrams,

3) tree diagrams [including fault tree diagrams],

4) prioritization matrices,

5) matrix diagrams,

6) process decision program (PDPC) charts, and

7) activity network diagrams.

iii. Control charts

Describes the objectives and benefits of control charts, including controlling process performance, identifying special and common causes, etc.

1. Rational subgrouping

Define and describe how and why rational subgrouping is used.

2. Populations versus samples

Understand the distinction between a population parameter and a sample statistic.

3. Random sample

Understand the basic questions that should be asked about whether or not a sample is to be considered ‘random’.

4. Analysis of control charts

Interpret control charts and distinguish between common and special causes using the rules for determining statistical control (i.e. seven points in a row for shifts and trends, points out of control, and non-random patterns).

iv. PRE-control chart

Define and explain PRE-control and perform PRE-control calculations and analysis.

b. Tools to address challenging situations

i. Advanced statistical process control

Understand appropriate uses of short-run SPC, EWMA, CuSum, and moving average.

ii. Design of experiments (DOE)

1. Terminology

Define independent and dependent variables, factors and levels, response, treatment, error, and replication.

2. Planning and organizing experiments

Describe and apply the basic elements of experiment planning and organizing, including determining the experiment objective, selecting factors, responses, and measurement methods, choosing the appropriate design, etc.

3. Design principles

Define and apply the principles of power and sample size, balance, replication, order, efficiency, randomization, blocking, interaction, and confounding.

4. Design and analysis of one-factor experiments

Construct these experiments such as completely randomized, randomized block, and Latin square designs, and apply computational and graphical methods to analyze and evaluate the significance of results.

5. Design and analysis of full factorial experiments

Construct these experiments and apply computational and graphical methods to analyze and evaluate the significance of results.

6. Design and analysis of two-level fractional factorial experiments

Construct experiments (including Taguchi designs) and apply computational and graphical methods to analyze and evaluate the significance of results; understand limitations of fractional factorials due to confounding.

7. Taguchi robustness concepts

Apply Taguchi robustness concepts and techniques such as signal-to-noise ratio, controllable and noise factors, and robustness to external sources of variability.

8. Mixture of experiments

Construct these experiments and apply computational and graphical methods to analyze and evaluate the significance of results.

9. Response surface methodology

10. Steepest ascent/descent experiments

Construct these experiments and apply computational and graphical methods to analyze the significance of the results.

11. Higher-order experiments

Construct experiments such as CCD, etc., and apply computational and graphical methods to analyze the significance of the results.

12. Evolutionary operations (EVOP)

Understand the application and strategy of EVOP.

c. Process capability and performance

i. Process performance vs. specification

Distinguish between process limits and specification limits.

ii. Calculate process potential

Calculate process performance metrics (Cp and Cpk).

iii. Process performance indices

Define, select, and calculate Pp, Ppk, and Cpm and assess process performance.

iv. Design for Six Sigma (DFSS) in the organization

Understand why DFSS is important to Six Sigma implementation. Understand robust design and functional requirements. Develop a robust design using noise strategies. Understand the concepts of tolerance design and statistical tolerancing. Calculate tolerances using process capability data.

d. Maintaining a solution

i. Standard operating procedures

Define methods for standardizing procedures.

ii. Error proofing/Mistake proofing/poka yoke

Define and apply methods/examples for error-proofing.

iii. Audits Define and apply methods for application.

iv. Project tracking

Define and apply methods for application.

When you’re ready, there are a few ways I can help:

First, join 12,000+ other Six Sigma professionals by subscribing to my email newsletter. A short read every Monday to start your work week off correctly. Always free.

* * *

If you’re looking to pass your Six Sigma Green Belt or Black Belt exams, I’d recommend starting with my affordable study guide:

1)→ 🟢Pass Your Six Sigma Green Belt: a study guide organized by the exam Body of Knowledge, 1,000+ practice questions with detailed answer explanations, and 8 full-length mock exams

2)→ ⚫Pass Your Six Sigma Black Belt: the full Black Belt Body of Knowledge, 2,000+ practice questions with detailed answer explanations, and 8 full-length mock exams

You’ve spent so much effort learning Lean Six Sigma. Why leave passing your certification exam up to chance? This comprehensive study guide offers 1,000+ exam-like questions for Green Belts (2,000+ for Black Belts) with full answer walkthroughs, access to instructors, detailed study material, and more.

​ Thousands have used this system to prepare. Join them here. 

X Bar S Control Chart

What are X Bar S Control Charts?

X Bar S charts often use control charts to examine the process mean and standard deviation over time. These charts are used when the subgroups have large sample sizes. Conversely, the S charts provide a better understanding of the spread of subgroup data than the range.

X bar S charts are also similar to X Bar R Control charts. The basic difference is that X bar S charts plot the subgroup standard deviation, whereas R charts plot the subgroup range.

Selection of an appropriate control chart is very important in control chart mapping, otherwise ended up with inaccurate control limits for the data.

Flowchart for selecting a control chart: continuous data routes by subgroup size to I-MR (n=1), Xbar-R (2 to 8), or Xbar-S (9 or more); discrete data routes to u, c, p, or np charts depending on whether defects or defectives are counted and whether sample size is variable or constant.

Manually, it is very easy to compute the X Bar R Control chart, whereas the sigma chart may be difficult due to tedious calculations and large sample sizes. With a large sample size in the subgroup, the standard deviation is a better measure of variation than the range because it considers all the data, not just minimum and maximum values.

It is actually two plots to monitor the process mean and the process range (as described by standard deviation) over time. Additionally, it is an example of statistical process control. These combination charts help to understand the stability of processes and detect the presence of special cause variation.

The cumulative sum (CUSUM) and the exponentially weighted moving average (EWMA) charts also monitor the mean of the process. But the basic difference is that, unlike the X bar chart, they consider the previous value means at each point. Moreover, these charts are considered a reliable estimate when a correct standard deviation exists.

X Bar S Control Chart Definitions

X-bar chart: The mean or average change in the process over time from subgroup values. The control limits on the X-Bar consider the sample’s mean and center.

S-chart: The standard deviation of the process over time from subgroups values. This monitors the process standard deviation (as measured by each subgroup’s sample standard deviation)

Notation Used in X-bar S Charts

  • m = Number of subgroups
  • n = Number of observations per subgroup
  • X̄ = Mean of one subgroup
  • X̄̄ = Grand mean (average of all X̄ values)
  • S = Standard deviation of one subgroup
  • S̄ = Average of subgroup standard deviations

For example, if you measure 5 parts each day for 25 days:

  • n = 5
  • m = 25

Use X Bar S Control Charts When:

  • The sampling procedure is the same for each sample and is carried out consistently.
  • When the data is assumed to be normally distributed.
  • The X bar S chart is to be used when rationally collecting measurements in a subgroup size of roughly 9 or more. Some references cite 10 as the cutoff, but the more precise guidance is that X bar S charts become preferable once the subgroup size reaches about 9, since the sample standard deviation becomes a more efficient estimator of variation than the range at that point.
  • X Bar R chart is to be considered if the subgroup size is between two and ten observations. For the I-MR chart, the subgroup size is one only.
  • When the collected data is continuous (i.e., Length, Weight), etc. and captures in time order.

How to Interpret the X Bar S Control Charts

  • To correctly interpret the X bar S chart, always examine the S chart first.
  • The X bar chart control limits are derived from the values of S bar (average standard deviation). If the values are out of control in the S chart, the X bar chart control limits are inaccurate.
  • If the points are out of control in the S chart, then stop the process. Identify the special cause and address the issue. Remove those subgroups from the calculations.
  • Once the S chart is in control, then review the X bar chart. Afterward, you should interpret the points against the control limits.
  • All the points are to be interpreted against the control limits but not specification limits.
  • If any point is out of control in the X bar chart, identify the special cause and address the issue.
Formula for estimating the process standard deviation once the S chart is in statistical control: sigma-hat equals S-bar divided by c4

Steps to follow for X bar S chart

Objective of the chart and subgroup size

  • Determine the objective of the chart and choose the important variables
  • Choose the appropriate subgroup size and the sampling frequency
  • Shewhart suggested collecting 20 to 25 sets of samples with a subgroup size of 10 and above

Note: To demonstrate an example, we just took subgroup size 4 in the below example. But, it is always recommended to take ten and above for the X bar S chart.

Example: A packing organization monitoring the performance of a packing machine, each container should weigh 35 lb, during the Measure phase, the project team performed the process capability study and identified that the process is not capable(less than one sigma). In Analyze phase collected 12 sets of container weights with a subgroup size of 4.

Table of raw measured values for the worked example: 12 samples with 4 measurements each

Compute X̄ and S values

  • Measure the average of each subgroup i.e X bar, then compute the grand average of all X bar values. This will be the center line for the X bar chart.
  • Compute the standard deviation of each subgroup, then measure grand averages of all standard values ie S bar, and this will be the center line for the S chart.
Spreadsheet computing the sample average and standard deviation for each of the 12 samples, arriving at grand mean 38.54 and S-bar 10.08

Determine the Control Limits

The first set of subgroups to determine the process mean and standard deviation. These values are to be considered for the creation of control limits for both the standard deviation and mean of each subgroup.

Diagram showing the sampling distribution of X-bar (narrower) versus the distribution of individual X values (wider), illustrating why control limits use the standard error rather than the raw standard deviation

The process is to be in control in the early phase of production.  Special causes are to be identified if any of the points are out of control during the initial phase. Also, the subgroup has to be removed for calculation.

A note on measurement system studies, because the X bar chart is read the other way around there. In a gage R&R study you plot each part’s average on an X bar chart, and the control limits come from repeat measurements of the same parts, so they show measurement error, not part-to-part variation. If the gage can tell parts apart, most of those points should fall outside the limits (AIAG’s rule of thumb is at least half). If most sit inside, the measurement noise is as large as the real differences between parts. On a production chart like this one, a point outside the limits is a special cause to investigate. See Measurement System Evaluation (MSE).

Derivation of the X-bar chart and S chart control limit formulas from the Shewhart 3-sigma rule
  • Where
    • X is the individual value (data)
    • n is the sample size
    • X̄ is the average of reading in a sample
    • S is the standard deviation
    • S bar is the average of all the standard deviations.
    • UCL is the Upper control limit
    • LCL is Lower control limit

X bar chart control limits:
UCL = X̄̄ + A₃ × S̄
LCL = X̄̄ − A₃ × S̄

S chart control limits:
UCL = B₄ × S̄
LCL = B₃ × S̄

Where A₃, B₃, and B₄ are control chart constants that depend on the subgroup size n; see the constants table below.

The below control chart constants are approximate values to measure the control limits for the X bar S chart and other control charts based on subgroup size.

Control chart constants table showing A2, A3, d2, D3, D4, B3, and B4 by subgroup size for X-bar, R, and S charts
Subgroup size (n) A2 A3 d2 D3 D4 B3 B4
2 1.880 2.659 1.128 – 3.267 – 3.267
3 1.023 1.954 1.693 – 2.574 – 2.568
4 0.729 1.628 2.059 – 2.282 – 2.266
5 0.577 1.427 2.326 – 2.114 – 2.089
6 0.483 1.287 2.534 – 2.004 0.030 1.970
7 0.419 1.182 2.704 0.076 1.924 0.118 1.882
8 0.373 1.099 2.847 0.136 1.864 0.185 1.815
9 0.337 1.032 2.970 0.184 1.816 0.239 1.761
10 0.308 0.975 3.078 0.223 1.777 0.284 1.716
11 0.285 0.927 3.173 0.256 1.744 0.321 1.679
12 0.266 0.886 3.258 0.283 1.717 0.354 1.646
13 0.249 0.850 3.336 0.307 1.693 0.382 1.618
14 0.235 0.817 3.407 0.328 1.672 0.406 1.594
15 0.223 0.789 3.472 0.347 1.653 0.428 1.572
16 0.212 0.763 3.532 0.363 1.637 0.448 1.552
17 0.203 0.739 3.588 0.378 1.622 0.466 1.534
18 0.194 0.718 3.640 0.391 1.608 0.482 1.518
19 0.187 0.698 3.689 0.403 1.597 0.497 1.503
20 0.180 0.680 3.735 0.415 1.585 0.510 1.490
21 0.173 0.663 3.778 0.425 1.575 0.523 1.477
22 0.167 0.647 3.819 0.434 1.566 0.534 1.466
23 0.162 0.633 3.858 0.443 1.557 0.545 1.455
24 0.157 0.619 3.895 0.451 1.548 0.555 1.445
25 0.153 0.606 3.931 0.459 1.541 0.565 1.435
Control chart constants (A2, A3, d2, D3, D4, B3, B4) by subgroup size, for X-bar, R, and S charts. Matches the table shown above.

Example cont: In the above example n=4

Worked control limit calculations for the container weight example: X-bar chart UCL=54.96, LCL=22.13; S chart UCL=22.85, LCL=0, using grand mean 38.54 and S-bar 10.08

Interpret X bar and S chart

  • Plot both the X bar and S chart and identify the assignable causes

Example Cont: Use the above values and plot the X bar and Sigma chart

X-bar chart and S chart for the container weight example, 12 subgroups plotted against the control limits; points outside the limits are circled in red showing out-of-control conditions

From both the X bar and S charts, it is clearly evident that most of the values are out of control. Hence the process is not stable

Monitor the process after improvement

Once the process stabilizes and control limits are in place, monitor the process performance over time.

Example cont: Control Phase – Once the process is improved and matured, the team identified the X bar S chart as one of the control methods in the Control plan to monitor the process performance over the time period.

The following are the measurement values in the Control phase of the project:

Table of raw measured values for the Control phase data: 12 samples with 4 measurements each

Compute X̄ and Sigma

Spreadsheet computing the sample average and standard deviation for the Control phase data, arriving at grand mean 34.58 and S-bar 1.43

Find the control limits

Recalculated control limits for the Control phase data: X-bar chart UCL=36.91, LCL=32.26; S chart UCL=3.24, LCL=0, using grand mean 34.58 and S-bar 1.43
X-bar chart and S chart for the Control phase data, 12 subgroups plotted against the control limits; the remaining out-of-control point (subgroup 2) is circled in red, showing the process has nearly stabilized

From the both X bar and S charts it is clearly evident that the process is almost stable. During the initial setup of 2nd data set both the S chart and X bar chart values are out of control, the team has to perform the root cause analysis for the special cause and also the process smoothing out from data set number 4. If that continued, the chart would need new control limits from that point.

  • Since the S chart is in statistical control, calculate the process standard deviation
Table of c4 factor values by subgroup size n (2 through 25), used to estimate the process standard deviation from S-bar
C4 Factors
Worked calculation of the estimated process standard deviation: sigma-hat = S-bar (1.43) divided by c4 (0.921) = 1.55
  • After the process is stabilized, if any point still goes out of control limits, it indicates an assignable cause exists in the process that needs to be addressed. This is an ongoing process to monitor the process performance.

Important notes on X Bar S Control Charts

  • A process is “in control” which indicates means of the process is stable, and it can be predictable.
  • A process is stable that does not mean it’s a zero-defect process.
  • Remember to NEVER put specifications on any kind of control chart.
  • The points on the chart are comprised of averages, not individuals. Specification limits are based on individuals, not averages.
  • The operator might tend not to react to a point out of control when the point is within the specification limits.
  • X bar S chart helps to avoid unnecessary adjustments in the process

X Bar S Control Chart Videos

https://www.youtube.com/watch?v=-O9Q4Z-nmfI

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Walter A Shewhart

Willam Shewhart
William Shewhart

Walter Shewhart is often called the “Father of Quality Control”. He worked as a process engineer for Western Electric and then at Bell Labs.  He retired in the mid ’50s. Juran actually worked under Shewhart.

We can trace Six Sigma as a measurement standard in product variation back to the 1920s when Walter Shewhart showed that three sigma from the mean is the point where a process requires correction.

Walter A. Shewhart Biography

Walter Andrew Shewhart was an American Physicist and statistician, and he is referred to as the ‘Father of Statistical Quality Control.’ Walter Andrew Shewhart was born to Anton and Esta Barney Shewhart in New Canton, Illinois, in the year 1891. He did his Master’s at the University of Illinois. Later, he attended the University of California at Berkeley to pursue a Ph.D. degree, where he received a Doctorate in physics in 1917. He worked as a professor at both universities. Then, he worked as head of the department of physics at Wisconsin Normal School, Lacrosse.

In 1918, Walter Shewhart worked at the Western Electric Company, one of the largest hardware manufacturers during that time. He assisted engineers in the manufacturing plant in refining the quality of telephone hardware. Later Shewhart went to work at Hawthorne until 1925; then, he worked at the Bell Telephone Research Labs. He would stay there until his retirement.

Walter Shewhart developed modern statistical concepts and scientific methods to minimize human efforts. Shewhart’s methods influenced other statisticians like W Edwards Deming and Joseph Juran. People often refer to these three people (Shewhart, Deming, and Juran) as the three authors of the quality upgrade movements. Shewhart’s work, specifically control charts and the PDSA cycle, influenced the daily work of quality extensively.

Reducing Variation: To Improve Quality

The emphasis on reducing variation to enhance quality is an excellent contribution to quality management. Reducing variation to improve quality resulted in the manufacture of precise things. The concept was applicable in different fields like automobiles, electronics, construction, etc.

Shewhart acknowledged two variation classes, namely ‘special‐cause’ and ‘common‐cause variation. These two categories can also be termed as ‘assignable‐cause’ and ‘chance‐cause’ variations respectively. He designed control charts to explain these two categories of variations. Shewhart proposed new attributes and variables in his control charts. Shewhart proposed to control common-cause to improve quality and reduce scrap. In this way, we can bring any process under statistical control. To distinguish between special cause and common cause variations, we must be sure to meet this criteria. After bringing a process to this state, it would be easy to forecast future outputs and manage processes economically. Shewhart’s principle paved the way for modern scientific analysis of process control.

Economic Incentives

The objective of any industry is to develop economical methods in order to satisfy human needs. We can do this by reducing things to routines requiring little human effort. By using scientific techniques and modern statistical theories, it was possible to set up limits for the results of routine efforts economically. We describe a routine as broken down and no longer economical if the results of any routine process deviate away from the limits. Consequently, we must find the cause of trouble, identify it, and eliminate it in order to make the process economical.

Six Sigma Concept

We use the Greek letter ‘Sigma,’ a Greek letter and mathematical term, to denote standard deviation. It is a standard statistical unit used to measure and describe the distribution of any process about its mean.

Shewhart’s ideas and statistical concepts were embraced in clinical laboratories for several years. Clinical laboratories used these concepts in proficiency testing and quality control operations. Many industries have rediscovered Shewhart’s methods and statistical process control tools, named ‘Six Sigma.’ Recently, Motorola Company has also made use of Six Sigma methods for improving product quality. Motorola Inc. was awarded the Malcolm Baldrige National Quality Award in the manufacturing category for using Six Sigma concepts for quality improvement. In recent times, the Six Sigma concept has acquired public attention extensively. Many other organizations also started using Six Sigma concepts, which has popularized it.

ASQ – American Society for Quality

ASQ of individuals who are passionate about methods of quality control. Members of ASQ contribute to the industry with their ideas of quality control and experience. Shewhart was ASQ’s first honorary associate, and he efficaciously brought together the principles of statistics, economics, and engineering. Individuals working in the field of quality control widely recognize his contributions, particularly because he developed highly effective tools specifically control charts.

Shewhart acquired appreciation in the statistical community for writing Statistical Method from the Viewpoint of Quality Control. Additionally, Bell Laboratories held a number of articles and journals that he had written, but fortunately, they published them later.

Another important element in Shewhart’s achievements was his quality of bringing out ideas and knowledge of other individuals.

Shewhart always believed that statistical theory would serve the needs of industry. After all, he was a man of science who worked patiently to develop ideas that made the world better. Shewhart’s contributions and ideas influenced ASQ intensely. Before his death, Shewhart mentioned to other members of ASQ that he appreciated their contributions. It was their efforts that led to extensive growth in the field of quality control. He also stated that the development, which was beyond his expectations, astonished him.