· Gage R&R studies what fraction of total variation comes from the measurement system (gage + person + method); it is a must-check for IATF 16949 / PPAP / system audits. Repeatability EV = variation of one person measuring the same part multiple times (the equipment itself); Reproducibility AV = variation across appraisers measuring the same part (the people)
· How to run: pick 5–10 parts spanning the tolerance range, label them, and have each appraiser measure them in random order blind, 2–3 repetitions each; this tool supports the AIAG Average Range (Xbar&R) method and the more rigorous ANOVA method, which also tests the "appraiser × part" interaction
· Acceptance: %GRR < 10% acceptable; 10%–30% conditionally acceptable (depending on application importance); > 30% unacceptable; ndc (number of distinct categories) should be ≥ 5
· Only after R&R is settled do CpK / control charts make sense