Is Your Process Stable?
For many of you, this is a basic reminder, but it's an incredibly important one. Before jumping into any DOE or regression study--or even most hypothesis tests--ask the question: Is your process stable? DMAIC is meant for solving common cause variation. If your process is not stable and in control, you have more than just common cause variation, you probably have special cause variation! Starting any type of DOE or regression analysis at this point may result in bad decisions. Why? Special cause variation often reveals itself in the form of outliers. Imagine running a DOE or regression study, and during your data collection, you get one of these outliers. What is that going to do to your results? How is that going to affect the significance of your factors? A factor might come forth as significant when it is really not. Be sure to address that special cause variation before looking at running a DOE or regression study. You can check your data with a simple histogram and normality test, a control chart, or even a chi-square test for attribute data.
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Often in a Six Sigma ( DMAIC) project the process starts off not stable.  One or more sources of special cause variation are present.  This highlights the need to apply SPC early in the project to identify them. Interesting that many training programs don't cover SPC until the control phase.  SPC had been a time tested tool and should be taught early in a Belt training program.