MSA (Gage R&R)
Repeatability & reproducibility — attribute and variable Gage R&R studies.
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Serves these standards
About this tool
Before you trust any measurement — a capability study, an SPC chart, an inspection result — you have to trust the measurement system itself. Gage R&R partitions observed variation into repeatability (same part, same operator, same gage) and reproducibility (between operators), so you know whether the gage or the people are the problem.
The standard acceptance yardstick from the AIAG MSA manual: %GRR below 10% of study variation (or tolerance) is acceptable, 10–30% is conditionally acceptable depending on importance, and above 30% the measurement system must be improved before the data means anything. For SPC use, the number of distinct categories (ndc) should be 5 or more.
Frequently asked questions
What %GRR is acceptable?
Under 10%: the measurement system is acceptable. 10–30%: may be acceptable for low-risk applications with customer approval. Over 30%: not acceptable — fix the gage or the method before drawing conclusions from the data.
Repeatability vs reproducibility?
Repeatability (equipment variation, EV) is the spread when one operator measures the same part several times with the same gage. Reproducibility (appraiser variation, AV) is the extra spread between operators. High EV points at the gage or fixture; high AV points at training, technique or unclear instructions.
How many parts, operators and trials?
The classic variable study is 10 parts × 3 operators × 3 trials, with parts selected to span the actual process range — not just reference standards. Randomise the measurement order so operators cannot remember earlier readings.
What is ndc (number of distinct categories)?
ndc estimates how many distinct groups the measurement system can reliably tell apart across the process spread. AIAG recommends ndc ≥ 5 for a system used to control a process; a low ndc with acceptable %GRR usually means the parts sampled too narrow a range.