Measurement variation itself has two parts, and the study is named after them. Repeatability is the variation you get when the same person measures the same part with the same gage several times. It is sometimes called equipment variation, because when one person does everything the same way, what is left is mostly the gage and the way it seats on the part. A caliper with worn jaws or a loose thumbwheel shows up here.
Reproducibility is the variation between different people measuring the same parts with the same gage. It is sometimes called appraiser variation. If one inspector squeezes the caliper jaws harder than another, or measures the hole at a different angle, their averages drift apart even when each person is individually consistent. Reproducibility is a people-and-method problem more than a gage problem.
Keeping the two straight matters because the fixes are different. Repeatability problems point at the gage, the fixture, or the part surface. Reproducibility problems point at training, technique, and the work instruction. The study separates them so you fix the right thing.
Standard Procedure
- 1Have each appraiser measure the same part at least twice; compute each person's range between trials.
- 2Compute each appraiser's average reading.
- 3Compare within-person ranges (repeatability) against the spread of appraiser averages (reproducibility).
- 4Note which of the two is larger before deciding what to investigate.
Lesson Summary
- ✓Repeatability points at the gage and setup; reproducibility points at people and method.
- ✓Both are computed from simple ranges and averages.