

29th June 2009, 11:29 AM


pValue(s) for Anova (Crossed) GRR (Gage R&R) Studies
Good day all,
I've had discussions with colleagues of mine here at work debating whether or not an acceptable pvalue is required to use a given set of data for a GRR study (10*3*3). I can't find where MSA or any other documented parameters require me to do so. The experience of one of my peers is that his customer (a Big3 automotive manufacturing company) required this in person, so we should have this as our standard accross the board.
Mathematically speaking, does an unacceptable pvalue (p>0.05) automatically render the data unacceptable for an ANOVAcrossed GRR, and if so, is the reason because it is an ANOVAbased study?
I appreciate any advise and/or thoughts you may have, and I apologize in advance for any breach in established posting protocol.
(Edit: I tried to copypaste the data, but was unsuccessful.)
Thanks!
Last edited by straetfeild; 29th June 2009 at 11:35 AM.

29th June 2009, 11:18 PM


Re: pValue(s) for Anova (Crossed) GRR (Gage R&R) Studies
I love this question. This is the first NEW MSA question that I have seen in a long time.
To start, the current MSA methodology as defined by AIAG, endorsed by the automotive industry and codified by the statistical software companies totally ignore the pvalues with one exception.
That exception is for the Operator x Part interaction. In Minitab, if the Op x Part interaction pvalue is greater than 0.25 (default value), the interaction is then pooled with the ANOVA error term thus becoming part of Repeatability. If it is less than 0.25, it is shown separately as a subitem under Reproducibility. Why 0.25? Minitab says to stay consistent with AIAG. AIAG wanted to be extremely conservative about pooling the interaction in error.
Technically, if the Reproducibility pvalue is greater than alpha (usually 0.05), it also should be pooled into the error term becoming part of Repeatability. It is only when it is less than alpha that Reproducibility can be distinguished from Repeatability. However, AIAG is silent on this matter and the canned MSA routines in software do not allow it. To do it yourself, you would have to run the standard ANOVA routines and manually generate the graphs.
The pvalue for Parts also provides information. If the pvalue of Parts is greater than alpha, the part variation is indistinguishable from Repeatability variation. If the pvalue is less than alpha, the gage can distinguish at least one part as different from the rest. Not definitive that the gage is good (p < alpha), but conclusive that it is not good (p > alpha).
Thank you for this question.

Thank You to Miner for your informative Post and/or Attachment!


5th May 2013, 03:22 AM


Re: pValue(s) for Anova (Crossed) GRR (Gage R&R) Studies
Quote:
In Reply to Parent Post by Miner
Why 0.25? Minitab says to stay consistent with AIAG. AIAG wanted to be extremely conservative about pooling the interaction in error.

Does it mean that 0.25 is another rule of thumb? Or maybe it has any statistical background? Sometimes I suspect that it has something to do with alpha level: because the interaction is part * oper, so it's like squaring 0.05 * 0.05, but it gives only 0.0025, not 0.25.

5th May 2013, 01:49 PM


Re: pValue(s) for Anova (Crossed) GRR (Gage R&R) Studies
It does have something to do with alpha. AIAG has arbitrarily established an alpha = 0.25. This means that there is a 25% chance that if the null hypothesis (there is no interaction) is correct you will mistakenly reject that hypothesis in favor of the alternate hypothesis (there is an interaction).
I strongly disagree with this approach. The alpha risk for the interaction should be the same as for the operators.

Thanks to Miner for your informative Post and/or Attachment!


21st October 2014, 02:04 PM


Re: pValue(s) for Anova (Crossed) GRR (Gage R&R) Studies
Good news for everyone. In Minitab 17 they lowered default "alpha to remove interaction" to 0,05 which is now explicable with CI :).
To Miner: In your previous message you say that AIAG requires 0,25 (I can't find a source of this information, whould you be so kind to give me the reference?). Does it mean that AIAG changed its mind and accepted recognized statistical approach?
Thank you for comments.

21st October 2014, 05:22 PM


Re: pValue(s) for Anova (Crossed) GRR (Gage R&R) Studies
I took a quick look through the MSA manual versions 3 & 4 and could not find anything either. I cannot remember whether this was in an earlier version and was dropped, but it appears to be a nonissue now.

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