Stop X-bar/R Madness!!
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Dr Wheeler's article discusses the calculations for XmR charting:Individual Charts Done Right and Wrong He notes that some software will allow you to pick algorithms that calculate control limits erroneously. My favorite quote from this article: "Thus, just as with average charts, there are right and wrong ways to compute limits for a chart for individual values. Because most software will provide you with the option of computing your limits incorrectly, it is your responsibility to determine how to get the right limits using your software." I might add for some charting, the correct control limit methodology is not available in the software at all, and you are on your own.