A gauge R&R study measures how much of the variation you see comes from the measurement system rather than the parts. Repeatability is the spread when one person measures the same part several times with the same gauge. Reproducibility is the extra spread when different people measure the same parts. A typical study has several operators measure the same set of parts two or three times each, in random order. The result is usually shown as a percentage of the tolerance or of the total observed variation.
Example: Three operators measure ten parts three times each. The study gives a combined R&R standard deviation of 0.0015 mm against a tolerance of 0.12 mm. %GRR of tolerance = 6 × 0.0015 ÷ 0.12 = 7.5%, so the gauge can be used to accept and reject parts.
A widely used rule of thumb treats under 10% as good, 10 to 30% as possibly acceptable depending on the use, and over 30% as not fit for the job. When R&R is high, the answer is often a better fixture, a clearer method or operator training, not a new gauge.
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