GRR studies will rarely represent the range of variation that you will encounter in production. The studies do a reasonable job of estimating measurement variation and it sounds like in your case this this will be the limitation.

One of the aspects which doesn't help is the way that AIAG type studies try to separate part variation from measurement variation. This is a topic which is too long to be talked about here, however if its important to your company to know how the measurement variation interacts with the part variation i recommend that you report on the Intraclass Correlation Coefficient (ICC).

Don't be put off by the title or by the fact that normal software doesn't compute this directly, the ICC can be computed straight from your normal GRR software by utilising ICC = 1- GRR squared/PV squared ........assuming of course that the measurement process is consistent.

This number is far more useful than trying to split out the PV like the AIAG studies do and it properly describes how measurement variation interacts with product variation.

In summary, if you have to describe how your measurements interact with part measurements and you have a choice choose ICC and give yourself at least a chance of proper representation. If the ICC is not what showing what you want then you will need a better estimate of part variation from somewhere, this is more likely to be true of the AIAG which will tend to overestimate the effects of measurement variation on the parts.

For more information on ICC see the reading room on Dr Donald Wheeler's website

www.spcpress.

Hope this helps somewhat