Six Sigma is a framework to make an organization more competitive by focusing on being effective and efficient. It is a systematic problem solving approach that is centered around defects elimination and variation reduction which leads to process improvement.

Six sigma is a continuous improvement process, with focus on:

  • Change empowerment
  • Seamless training of resources
  • Consistent top management support

Six sigma refers to the methodology and practice of focusing on developing and delivering products/services that perform at high standards. It is a Quality philosophy and a management technique. Six sigma is a not a standard or a certification or another metric like percentage.

Six Sigma’s Basic Premises

The basic purpose of six sigma is to delight the customers; this can be achieved by delivering the quality product. Hence it is fundamentally focuses on variation reduction and waste elimination that ultimately lead to increased efficiency.

Variation is the range of difference between the statistical mean and all data points that are used to calculate the mean. In other words the extent which process performance varies around the mean.

 Waste or defect is a measurable product characteristic that does not meet a customer requirement.(defective products).In other words, any process performance outside of defined specification.

Six sigma methodologies used to identify and eliminate the root cause of defects or minimizing the process variation (using statistical or process analysis methods) and also the costs that are associated with it.

Using a manufacturing example, let’s say XYZ factory that produces widgets.  The widgets are all supposed to be a certain size, shape, and weight.  Randomly sampled output discover that out of every 1,000 widgets fifty are flawed (e.g. wrong size, shape, or weight).  To correct this factory use Six-Sigma to determine what’s causing the variation/defects and work to reduce them to a six-sigma level.

Six Sigma’s Approach to Problem Solving

Six Sigma was originally coined by Bill Smith. He was an American engineer and also known as father of Six sigma. It was first implemented in Motorola in the year 1986 as a general approach to measuring quality in business performance terms. Further, it becomes popular management approach at GE with Jack Welch in early 1990s’.

General Electric reported tangible benefits of $2.5 billion per year due to Six Sigma initiatives and made it an organization wide strategy. Now it has evolved be a project driven approach to process improvement that follows the five-step process also called DMAIC cycle. (Also see: History of Continuous Improvement).

Overview Video

What is a Six Sigma Level?

Six Sigma derives from the normal or bell curve in statistics, where each interval indicates one sigma or one standard deviation. Moreover Sigma is a statistical term that refers to the standard deviation of a process about its mean. In a normally distributed process, 99.73% of measurement will fall within ±3σ and 99.99932% will fall within ±4.5σ.

For a process, the sigma capability is a metric that indicates how well that process is performing. Hence higher the sigma value, better the process.

The 68-95-99.7 rule also known as empirical rule used to remember the percentage of values that lie within a band around mean in a normal distribution with a width of two, four and six standard deviation respectively.

In the above example the defect rate is 50/1000.  Where as 6σ quality is 3.4 / 1,000,000.

A six sigma capable process will have Cp = 2.0, Cpk = 1.5 and DPMO = 3.4

Where Does the 6 and Sigma come from?

Sigma denoted by symbol σ (a Greek alphabet) represents standard deviation of a population. Primarily it characterizes the dispersion of a set of data values with respect to mean. It refers to the mathematical concept of standard deviation.

What is Six Sigma

In a normally distributed process if mean ±3σ range of output is between USL and LSL then around 99.997% area of the points of the output will be non defective. In other words one can say the process is in six sigma level. The 6 refers to the ability to fit the good outputs of the process to fit within 6 standard deviations (or sigmas). Another way to say it is that only 3.4 defects occur per every million opportunities fall outside the control limits.

The Primary Goals are to Improve

Six Sigma is a data-driven methodology it contains statistical tools and techniques to define the problem and evaluate each step of a process. Six Sigma provides ways to improve efficiencies in a business structure and also improves the quality of the organization processes and increases the bottom-line profit.

  • How effective the process meeting or exceeding customer’s needs and requirements.
  • How efficiently the process operates.

Three Key Elements of Six Sigma

Unlike other quality improvement philosophies, Six Sigma is comprised of three key elements:

  • Customer: Customer is the key for business and they are the top priority in Six sigma. Customers defines the Quality and also expect on-time delivery, high performance, service and many more. However, meeting customer requirement is not sufficient in this competitive world, needs to delight them.
  • Process: Defining the process and corresponding metrics is the key aspect in six sigma. Since customer is key for any business, Quality needs to be looked from customer perspective. This will help to identify the gaps in processes and work to improve them.
  • Employee: Without leadership commitment it is difficult to implement Six sigma in any organization. Organization must involve all employees with well defined roles and clear objectives in a six sigma program. Furthermore organization needs to provide required resources (like people, training, budget) etc.
What is Six Sigma

Philosophy & Methodology

Six Sigma is a result-oriented, continuous improvement process that includes active involvement from senior management.

Continuous improvement activities in an organization ensure to identify and solve the problems as and when they occur. Most of the modern quality improvement models such as sampling techniques, control charts, process capability and DOE, etc. have been influenced by the theory of continuous improvement.

Why Use Six Sigma?

Organizations face new challenges everyday like raising cost, customer issues, increase in defect rate etc. Most importantly global competition made it imperative to provide near perfect quality at low cost to keep customer delighted and make organization viable in the market place. Following are the key benefits from Six Sigma implementation

  • Creates robust products and services
  • Ensures customer satisfaction
  • Reduces process variation and also eliminate waste
  • Eliminates the root causes of problems and defects in a process
  • Reduces rework by getting it right the first time
  • Addresses key organization business requirements
  • Competitive advantage

Six Sigma Methodologies

Six sigma provides better value to customers, employees and all the stake holders in an organization. Additionally, it is a business initiative which improves quality, productivity and reduces cost.


One of the chief tools in Six Sigma is the use of the DMAIC methodology. (Also see DMAIC Overview). Particularly, DMAIC is a logical framework that helps you think through and plan improvements to a process in pursuit of achieving a Six Sigma level of excellence. In other words it is a data driven quality strategy for improving the process.

There are five phases that are used in the DMAIC method. They are as follow:

  • Define: Define the problem and also project goals that needs to be addressed.
  • Measure: Measure the baseline performance and validate the practical problem by collecting data
  • Analyze: Analyze the data to determine the root causes of the problems and opportunity for improvement
  • Improve :Improve the process and test the statistical solution.
  • Control: Start using a new process for production. Implement, control and ensure the sustainability. Finally, measure the new process by based on customer reactions.

Along the way six sigma team to define the problem, work with sponsors and stakeholders, build and organize a team, and also deliver results that will be apprised at phase-gate meetings called DMAIC Tollgate Reviews.


DMADV is one aspect of Design for Six Sigma (DFSS). In fact this was evolved from the continuous quality improvement and Six Sigma approach to reduce variation. However, it is typically used to create new processes and new products or services.

  • Define: Determine the project need and also identify the goals
  • Measure: Asses customer needs and prioritize requirements
  • Analyze: Analyze data, generate various designs and also select concepts
  • Design: Optimize design components and completes the design
  • Verify: Validate and quantify the design performance.

Amusing: How this conversation usually goes:

The Basics Videos

Additional Articles

Six Sigma Tools Walkthrough

 Roles & Responsibilities

Six Sigma Black Belt Certification Questions:

Question: The primary reason that most companies implement six sigma is to: (Taken from ASQ sample Black Belt exam.)

(A) reduce defects
(B) improve processes
(C) improve profit
(D) increase customer satisfaction

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Comments (53)


Is there any pre-requisites for enrolling for LSS? I have 13 yeasr of experience in BPO’s and Call Centers and thinking of movie to some other verticals but for that I need to go for certain certifications then I came across your site. So thought to better consult so that you can guide me in the better direction.

Hi Raj,

There are no prerequisites for enrolling in Six Sigma training other than a will to learn!

Different certifying organizations do have different requirements on what they require to let you be certified though.

Best, Ted.

How do I take six sigma exam from Villanova university?can you comment on Moresteam and Villanova university exam.can we take the the exam in parts or at the sam grime.thankig you.Regrds.Dr.K.Kumar

Hi Dr. Kumar,

You can take the Villanova exam by registering for their Six Sigma Black Belt course. The exam comes after 6-8 weeks of training (if memory serves) and after you complete the example project.

I ma not personally familiar with Moresteam but some colleagues have recently used their training to prepare – and pass – the ASQ Six Sigma Black Belt exam.

Hope that helps!


i am interested in pursuing lean six sigma black belt. After some research I ended up with ASQ , LSSA (APMG), IASSC.

appreciate if you could guide me on selecting one of them. I am struggling to select one of them or understand the differences between them .


There is an error in the diagram of six sigmas, it only shows three sigmas. (99.5%) Six sigmas is six standard deviations on a single side of the mean.

I was under the same impression, Six Sigmas on either side, otherwise you end up with 5 bad parts in 1000, not the 3.4 DPMO. I have a hard time finding this information elsewhere on the internet.

Can you elaborate on why it is 3 sigmas on either side and not 6?

Short answer is that the center of curve is the mean of the process. 6 sigmas represent nearly all of the possible outcomes. If the center is the mean, you’d expect half of the possibilities to fall on either side (in a normal distribution.) Half of 6 is 3, thus 3 sigmas on each side of the center.

That’s not true…A six sigma process operates with Cp=2
So the natural tolerance needs to be 6 sigma on either side of the mean.

Hi, I am new to Six Sigma but have lots of experience in working on lean projects. How do i start with six sigma . Is yellow belt certification from ASQ is the right way to start. Is there any study materials you suggest.

Hi Steven,

I’ve really seen no use in pursuing yellow belts. Perhaps it’s a good way to quickly get recognized for knowledge you already have and add a resume item, but even then I’ve not seen employers value them. Perhaps other people’s experiences are different.

I think the right way to address your question is to begin with the end in mind. What is it that you want to accomplish. If you want to be able to effectively participate in lean six sigma projects and begin to get an understanding of the whole, I’d pursue the green belt. If you are looking to be a change agent and lead six sigma projects, I’d pursue Black Belt training and certification.

Either way, I think this website has the best available (and free!) study guides. Start by answering the survey here – it will help me provide exactly what you are looking for and will sign you up for our free email series that will logically lay out a course of action.

Remember, you can always see what the ASQ Green Belt exam or the ASQ Black Belt exam entails by clicking those links.

Hi! Dan,
I am little bit confuse about this answer which says that improve profit would be the primary goal to adopt six sigma. We know sigma is directly related to DPMO ( it talks about defect which due to process variation ) so in this sense ans should be A.

Hi Ajay, we’re talking about 2 different things here. For a company or an enterprise to adopt the philosophy of Six Sigma the biggest driver is improving the bottom line. If you want to calculate the Sigma of a process, you would calculate DPMO. Make sense?

I seem to agree with Ajay on the aspect of confusion,but on my part “philosophically” I see as if part D, customer Satisfaction. In this sense,I’m looking at a customer not only from buying and selling perspective but also for non profit entities.

Try thinking about profit in terms of it being the single biggest driver of being able to keep a company afloat.

If that’s not the case, say for a non-profit, I’d substitute it for something that is. To my mind non-profits cannot work without funding. So in that case replace with whatever can secure that non-profit funding.

Also, there are many, many non-profits that make tremendous amounts of money. That money allows them to serve their mission. However, without the money there is no serving a mission.

I would look at the profit being the dependent variable and the rest independent variables.
I would look at it as the direction of the Cause and Effect diagram

Hi Ted
Glad to see you chose a video by Beth Cudney. She facilitated the SSGB course at the University of Syracuse with IVMF. Ok, pushing forward…

Hi Sharon,

I had the chance to see Beth at the 2017 ASQ World Conference. She was presenting her method of teaching if I remember correctly. Sounded like her approach would be effective!


There is a lot of back and forth with the above question regarding how many sigmas to the right and left of the center point.

“There is an error in the last diagram. 6 sigma means +/- 6 sigma which means 6 deviations on either side.”

Is it + or – 3 or + or – 6?

Is that really a question from ASQ exam? because it seems so dark and inadequate… there is obviously no right answer to the question. How can i verify it is indeed a real question? because without verification it makes me feel all this page has fake information. Thank you!

Hi Ted,
I am an electrical engineer and i have done my PMP. I have seen the project requirements for ASQ and they lean more towards quality /accounting and production. I am into construction and we still apply Six Sigma but not to that extent as we would in manufacturing. I still want to get my black belt. I dont have manufacturing or production projects but do have construction projects. in my case which one do you recommend IASSC or ASQ.

Thanks for your help.

Hi Zoya,

I think that Six Sigma is appropriate to many, many industries, not just manufacturing. For example, I was able to earn black belt from two different organizations while working in financial software – ostensibly a service-based industry.

I think the choice between IASSC and ASQ comes down to what you want out of your certification, the kind of test-taker you are, and, at the black belt level, if you have a portfolio of accomplished projects or not. I have more on this topic here.

Best, Ted

Based on the Table (which can be confusing), 3.4 defects per million opportunities is actually 4.5 sigma. This is considering all “6 sigma” process over time would exhibit a drift (of 1.5 sigma) over a period of time (Long Term).

Hi Derek,

Thanks for your comment. Can you clarify? The table I have above has one section with a 1.5 shift and another without.

If you can tell me where it is confusing, I can make another that is less.

Best, Ted.

Hi David,

It’s project-specific. But there’s some vocabulary to unpack here, too.

Sigma is a measure of variation, true. As you reduce variation of your process your Sigma level increases. If you get to a 6 Sigma level, you have very low variation in your process.

The 6 Sigma process is generally referred to as DMAIC, which is a structured problem solving framework where you solve a business issue. Solving that business issue might be changing the mean, median, mode of a process. It might be reducing variation and increasing your sigma level (possibly even to a 6 sigma level). It could be used to do many things. The guiding principal is solving the business problem you are chartered with solving.

Best, Ted

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