Gage R&R Study

What it tells you

A Gage R&R (repeatability and reproducibility) study tells you how much of the variation you see comes from the measurement system rather than from the parts. Several appraisers measure the same parts several times. Repeatability (EV) is the variation when the same appraiser measures the same part again; reproducibility (AV) is the difference between appraisers.

Run it before you trust a control chart or a capability study: a noisy gauge can hide real changes or create false ones. A common layout is 3 appraisers, 10 parts covering the process range, and 2 or 3 trials.

Method

Both methods of the AIAG MSA manual, 4th edition, are available: Average & Range and ANOVA. The study gives %GRR of the total variation and, when you enter the tolerance, of the tolerance, the number of distinct categories (ndc), and a verdict against the manual’s acceptance guidelines. The calculation reproduces the worked example printed in the manual.

Worked example

The worked example printed in the AIAG MSA manual: 3 appraisers, 10 parts, 3 trials each, no tolerance.

Both methods put the gauge in the “may be acceptable” band, depending on the application. Reproducibility is slightly larger than repeatability, so appraiser training or a clearer measuring method is the first place to look.