Hi,
I am currently working on a Cpk Ppk analysis (JMP Pro 17.1) and I am very unclear on what subgrouping is and whether it is needed in my analysis. I have the same lab results dataset however did the Process Screening/Capability slightly differently:
1. Process Screening --> Process Variables = Grade A (leaving Grouping, Subgroup, Time and By column empty).
Result:
2. Process Screening --> Process Variables = Grade A, Time --> Year Quarter (leaving Grouping, Subgroup and By column empty).
Result:
Method 1 control chart defaults to IMR control chart whereas method 2 default to Xbar control chart which I'm guessing is the reason for the difference in Cpk. However, I do not understand why that 2 different controls are used in this and which would be more accurate for my analysis since there's quite a large differences in the Cpk numbers. Would love to get any insight on this.
Would also appreciate if someone would help to explain what N subgroup actually mean in my analysis here (is it averaging my data set each quarter?).
Thank you.
Hi @ConditionalAvg2 ,
You are right about Method 1 & 2 (further reading: https://www.jmp.com/support/help/en/17.2/index.shtml#page/jmp/overview-of-the-process-capability-platform.shtml )
I would prefer using Ppk (if you noticed, regardless of with/without subgroup the value is the same because it considers the whole dataset standard deviation).
Whole Statistical Thinking for Industrial Problem Solving (STIPS) course that is offered (for free) is great, but if I remember correctly (I did STIPS two or three years ago) Quality Methods parts did have good discussion regarding subgroups and selecting them. I think you can just take single module if you want to (though I think it is recommended to do them in order).
Statistical Thinking for Industrial Problem Solving - Quality Methods
