Normality is Overrated
The Six Sigma community seems to be obsessed with whether their data is normal or non-normal. If you’re using the data for control charts, normality is overrated.
Donald J. Wheeler, author of Advanced Topics in Statistical Process Control, just published an insightful paper on what he calls, Leptokurtophobia–the abnormal need for normal distributions–at http://www.qualitydigest.com/inside/quality-insider-column/do-you-have-leptokurtophobia.html.
Wheeler says: “We do not need to check the data for normality, nor do we need to define a reference distribution prior to computing limits. Anyone who tells you anything to the contrary is simply trying to complicate your life unnecessarily. ”
“Transforming the data prior to using them on a process behavior chart is not only bad advice, it is also an outright mistake.”
