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Between Sigma Calculation Mismatch issue in 3-way Control Chart
Hello,
The Between Sigma in 3-Ways Control Chart is 0.033745
In JMP document, the between sigma calculation is:
Accepted Solutions
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Re: Between Sigma Calculation Mismatch issue in 3-way Control Chart
in your excel sheet your between formula is calculating something different.
=(G7^2-O3^2/6)^0.5
I'm not sure where the subgroup means are calulated, I can see the subgroup ranges but of course that's something different too.
I used this formula to replicate the calculation in JMP
Sqrt( Col Mean( :Moving Range of Subgroup Means / :d2 2 ) ^ 2 - :Within Sigma ^ 2 / (60 / (60 * (1 / 6))) )
The harmonic mean is represented with this expression rather than writing the whole thing out. (60 / (60 * (1 / 6))), its still 6 though.
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Re: Between Sigma Calculation Mismatch issue in 3-way Control Chart
in your excel sheet your between formula is calculating something different.
=(G7^2-O3^2/6)^0.5
I'm not sure where the subgroup means are calulated, I can see the subgroup ranges but of course that's something different too.
I used this formula to replicate the calculation in JMP
Sqrt( Col Mean( :Moving Range of Subgroup Means / :d2 2 ) ^ 2 - :Within Sigma ^ 2 / (60 / (60 * (1 / 6))) )
The harmonic mean is represented with this expression rather than writing the whole thing out. (60 / (60 * (1 / 6))), its still 6 though.
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Re: Between Sigma Calculation Mismatch issue in 3-way Control Chart
Thanks Byron!
I think the root cause is that I didn't use mean of moving range on means as MR for the calculation.
I used individual MR (n=2) instead which caused the Between Sigma results mismatched.
Do you know why we need mean of moving range on means? Do you know the realistic explaination behind this calculation? Appreciate it.
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Re: Between Sigma Calculation Mismatch issue in 3-way Control Chart
So, two answers.
1. Control chart theory is pretty important. I like the Donald Wheeler text books for the practical approach, also Douglas Montgomery has a more mathematical and theoretical text book that is very good too.
2. If you trying to "validate" JMP with Excel, your're likely to run into some issues with Excel.
JMP's numerical accuracy is vetted extensively, and all the methods in JMP, especially in the quality tools, are from referenced sources.
quick answer:
Just looking at the between subgroup variation isn't enough. It might look consistent and be hiding within subgroup variation.
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Re: Between Sigma Calculation Mismatch issue in 3-way Control Chart
Thank you so much, Byron! Your feedback and comments helps me a lot.