topic Control charts in Discussions

Control charts

cesarpc98 — Sun, 11 Jun 2023 11:16:27 GMT

Hi, I would like to know if in order to make a Xbar-R chart, my data has to be normally distributed?

Re: Control charts

Jeff_Upton — Tue, 27 Jul 2021 15:11:27 GMT

Your data does not need to be normally distributed to use an Xbar-R chart. An important concept to address with this sort of chart is how you are subgrouping your data, referred to as rational subgrouping.

If you'd like to learn more about subgrouping and normality then you can check out the Quality Methods module of JMP's free Statistical Thinking for Industrial Problem Solving course. There are also quite a few posts to peruse on these topics in the JMP User Community in case you'd like to benefit from your predecessor's questions.

Welcome to the Community!

Jeff

Re: Control charts

statman — Tue, 27 Jul 2021 17:11:07 GMT

Adding to Jeff's response, I suggest you read Dr. Wheeler's papers/books regarding the incorrect assumption of normality when using control charts (Shewhart's charts).

Wheeler, Donald, and Chambers, David (1992) “Understanding Statistical Process Control” SPC Press (ISBN 0-945320-13-2)

You can also read Shewhart's original book:

Shewhart, Walter A. (1931) “Economic Control of Quality of Manufactured Product”, D. Van Nostrand Co., NY

Bill Woodall also has an excellent paper:

Woodall, William H. (2000), "Controversies and Contradictions in Statistical Process Control", Journal of Quality Technology, Vol. 32, No.4 October 2000.

Also go here:

https://community.jmp.com/t5/JMPer-Cable/Designing-Insightful-Process-Behavior-Charts/ba-p/21332