What is Graphical Analysis?
Graphical analysis is one of the best ways to analyze the problems in Six Sigma projects. It is an effective way to visualize data patterns and provides key insights into the data.
Generally, in any manufacturing or business process, six sigma teams deal with a vast amount of data, and it is not practically possible to convey the information in an effective way with the raw data. Thus, it is recommended to segregate and plot the data to analyze the problems.
The graphical analysis creates pictures of the data, and this will helps to understand the patterns and also the correlation between process parameters. Often graphical analysis is the starting point for any problem-solving method.
Graphical tools are readily available in Excel and in statistical software such as Minitab, and they are very easy to use and can quickly plot the graphs.
Walkthrough of Graphical Analysis Tools
Different graphical tools depict different features of the data such as trend, frequency, dispersion and shape of the distribution. Below are the few graphical analysis tools.
Box-and Whisker Plot
Box-and-Whisker plot is also known as Box and Whisker plot, is a pictorial representation of continuous data. Box plot shows the Max, Min, median, interquartile range Q1, Q3, and outlier.
Run chart is also known as time series plot, it is a line graph of data plotted over time. It helps to identify the pattern of the data in the time series. Because they don’t use control limits, we cannot judge the process is stable or not. However, run chat shows how the process is behaving. The advantage of the run chart is to identify the special cause(s) in the process.
Scatter diagram also known as correlation chart or XY graph. A scatter diagram plots the relationship between two continuous variables. Such as independent variable in the x-axis and the dependent variable on the y-axis.
Histogram is the graphical representation of a frequency distribution. It is in the form of a rectangle with class interval as bases and the corresponding frequencies as heights. Particularly, there is no gap between any two successive rectangles.
Normal Probability Plot
The Normal probability plot is a graphical method to assess the data set follows a normal distribution or not. This includes identifying outliers, skewness etc. Furthermore, Normal probability plot is one example of quantile-quantile (Q-Q) plot.
Pareto Chart is also known as 80-20 rule. It is a combination of bar chart and a line chart. The actual data in descending order using bar chart and cumulative data in ascending order using a line graph.
Bar Chart displays the frequency on one axis and the values of the categorical variable on the other axis. In a bar graph, bars of uniform widths are drawn with various heights. However, the height of bars represents the frequency of the corresponding observation.
Why Use Graphical Analysis Tools
Six Sigma teams applies a wide range of graphical tools in each phase of DMAIC.
The main purpose of the Define phase is to define the problem, identify customer requirements, and also summarize the project plan.
Project team uses various six sigma tools in Define phase like SIPOC (Supplier-Input-Process-Output-Customer), Process maps, value stream mapping, Project charter, SWOT analysis and Voice of customer.
Six Sigma team uses graphical analysis tool. Example Bar Chart for process trend, revenue loss etc. Run chart to monitor customer complaints or defects over a period of time. Similarly, Box plotis to graphically represent the voice of customer (pictorially depict customer satisfaction of various attributes).
Measure the process performance. In fact, the main purpose of the measure phase is to collect the data that covers the project scope and also to determine what data distribution you are working with.
Project team uses various Six Sigma tools like process flow chart, Gage R&R, Pareto chart, process capability analysis in the measure phase.
Graphical tools like Pareto Chart is to analyze the frequency of problems and identify the majority (80%) of issues. Similarly, the process capability analysis is to assess the ability of process performance according to the specification.
The main purpose of Analyze phase is to understand the root cause of the problem. Furthermore to identify the opportunity for improvement.
The main purpose of Improve phase is to eliminate the root causes and implement the improvements. Additionally, design the action plan to monitor the continuous improvements.
The main purpose of the control phase is to validate that post-improvement samples follow the designed and expected results.
In fact, graphical tools like statistical control charts (I-MR chart, u-chart, p-chart, and c-chart) helps to monitor progress and to make necessary adjustments.
How to Pick Which Tool to Use?
The selection of graphical tools depends on the type of data and also the objective of the project. The below picture helps to identify the right graphical tools based on the scenario. However, statistical analysis is required before drawing any conclusions.
How to Understand what the tools are telling you?
Often, the biggest challenge for Six Sigma teams to understand the graph and make conclusions. In fact, it is difficult to understand the message from the graph right away. Six Sigma team has to take time to interpret the graphs.
Each graph provides different messages like trend, frequency, dispersion, and shape of the distribution.
- Box-and-Whisker plot is to understand the difference between the groups, outliers in the process, maximum and minimum values.- Is there any difference between subgroups median?
- Histogram is to understand the shape of the distribution- Is it symmetrical or skewed?
- Scatter diagram is to understand the relationship between two continuous variables.- Is there any correlation between factors?
- Time series plots are to understand the trend over the time.- Is the process stable over time?
- Pareto Chart is to understand the frequency of the problem as well as their cumulative impact.