Gage R&R Analysis: %GRR Acceptance Criteria and Online Calculation
What is Gage R&R?
Gage R&R (GRR) is the core tool of Measurement System Analysis (MSA). It decomposes measurement variation into repeatability EV (the spread when one operator measures the same part repeatedly with the same gage) and reproducibility AV (the difference between operators' averages on the same parts), which combine into GRR. GRR is then combined with part-to-part variation PV into total variation TV, and %GRR = GRR/TV × 100% expresses the measurement contribution; the tool also reports the tolerance-based %P/T.
When to Use It
Use it when qualifying a new gage, improving a measurement system, preparing PPAP and control plan submissions, or before a process capability study where measurement error would distort the result. It is also the expected evidence in MSA audits. The classic design is 10 parts × 3 operators × 3 trials, with parts covering the process tolerance range.
How to Use It (Step by Step)
Select 10 parts that represent the actual process variation, have 3 operators measure each part 3 times in random order, and keep operators blind to part numbering to avoid memory bias. Paste the data into the tool and choose the Xbar-R or ANOVA method. The tool automatically computes %GRR, %P/T and ndc and gives the acceptance verdict with improvement hints for failing items.
Key Formulas / Example
%GRR = GRR/TV × 100% and ndc = 1.41 × (PV/GRR). Per AIAG MSA 4th edition: %GRR under 10% is acceptable, 10% to 30% is conditionally acceptable depending on application, cost and risk, and above 30% is unacceptable. ndc must be 5 or more; below 5 means the gage cannot distinguish between parts.