Careful here … you are mixing up on the indices.
The AIAG MSA Reference Manual uses the following accpetance criteria for %GRR and %Tolerance. Note that these are standard deviation-based metrics:
<10% – generally acceptable
10%-30% – may be acceptable based upon importance of application, cost of measurement device, cost of repairs, etc.
>30% – not acceptable
Use %GRR when the measurement will be used for process control situations (SPC, process capaiblity, and process improvement)
Use %Tolerance when the measurement will be compared to a specification (inspection or end-of-line comparison to specs).
For a capable process (where Pp>1) the %Tolerance will always be smaller than the %GRR, since the Tolerance width is larger than the 6sigma width. Note that AIAG MSA 3rd Edition now uses 6.00 as the multiplier rather than the old 5.15.
The %Contribution, on the other hand, is a variance-based metric.
AIAG does not specify acceptance criteria for %Contribution. It is usually used to provide an understanding of the respective sources of variation. If %GRR or %Tolerance are too high, you can use %Contribution to determine if the problem is with the operators or the gage itself.
Regardless, %Contribution is actually the square of the respective %StudyVar. So to convert the standard deviation-based acceptance criteria for use with %Contribution we take the square of the cutoffs:
<1% – generally acceptable (.10^2 = .01)
1%-9% – may be acceptable based upon importance of application, cost of measurement device, cost of repairs, etc.
>9% – not acceptable (.30^2 = .09)
Again, these squared metrics are ONLY for %Contribution.
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