What is the difference between repeatability and reproducibility?

Repeatability is variation when the same appraiser measures the same part with the same gauge — it is equipment variation. Reproducibility is the variation between appraisers measuring the same parts — it is appraiser variation. High repeatability error points at the gauge; high reproducibility error points at training, method or fixturing.

Home › Tools › Quality & SPC

Gage R&R Calculator

Gage R&R quantifies how much of your observed variation comes from the measurement system rather than the parts. If the gauge consumes a large share of the tolerance, every capability study, control chart and accept/reject decision built on it is unreliable — which makes MSA the correct first step before any SPC work, not an afterthought.

Open the Gage R&R Validator tool iOS app Android app

Formula

Combined gauge variationσ_GRR = √(σ²_repeatability + σ²_reproducibility)

%GRR against study variation%GRR = 100 × σ_GRR / σ_total

%GRR against tolerance%GRR = 100 × (6 × σ_GRR) / (USL − LSL)

Number of distinct categoriesndc = 1.41 × (σ_part / σ_GRR)

Truncated to an integer. AIAG requires ndc ≥ 5.

Worked example

A study returns σ_repeatability = 0.08, σ_reproducibility = 0.05, σ_part = 0.42. Tolerance is 1.20.

σ_GRR = √(0.08² + 0.05²) = √(0.0064 + 0.0025) = √0.0089 = 0.09434
σ_total = √(0.09434² + 0.42²) = √(0.0089 + 0.1764) = √0.1853 = 0.43046
%GRR (study variation) = 100 × 0.09434 / 0.43046 = 21.9%
%GRR (tolerance) = 100 × (6 × 0.09434) / 1.20 = 47.2%
ndc = 1.41 × (0.42 / 0.09434) = 1.41 × 4.452 = 6.28 → 6

Against study variation the gauge looks marginal at 21.9%; against tolerance it is clearly unacceptable at 47.2%. This gap is common and matters: if the gauge is used to accept or reject parts against a specification, the tolerance-based figure is the one that governs.

Run this with your own numbers

How to interpret the result

%GRR < 10%	Measurement system acceptable.
%GRR 10–30%	May be acceptable depending on application, cost of the gauge and criticality of the characteristic.
%GRR > 30%	Unacceptable. The measurement system needs improvement before its data is used for decisions.
ndc ≥ 5	The gauge can resolve enough distinct part levels to be useful for process control.

Common mistakes

Reporting only %GRR against study variation when the gauge is used for conformance decisions. Selecting parts with unusually wide spread inflates σ_part and flatters the result.
Using parts that do not span the expected process range — the study then measures the gauge against an artificial part population.
Letting appraisers see previous readings, or measuring in a fixed non-randomised order.
Ignoring ndc when %GRR passes. A gauge can show acceptable %GRR yet still resolve too few distinct categories to control the process.

Frequently asked questions

What is the difference between repeatability and reproducibility?: Repeatability is variation when the same appraiser measures the same part with the same gauge — it is equipment variation. Reproducibility is the variation between appraisers measuring the same parts — it is appraiser variation. High repeatability error points at the gauge; high reproducibility error points at training, method or fixturing.

About Reliability Pro

Gage R&R Validator is part of a suite of 126 reliability, maintenance and quality engineering tools covering life data analysis, accelerated testing, system reliability, FMEA and root cause, SPC, and design for reliability. It runs in the browser and as native iOS and Android apps, so the same calculation is available at a desk or in front of the asset.

Browse all 126 tools Open the web app