Gage R&R
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Gage R&R (Repeatability and Reproducibility) is a study that quantifies measurement system variation to ensure data quality for process improvement.
Definition
Gage R&R (Repeatability and Reproducibility) is a statistical study that quantifies how much observed variation comes from the measurement system versus the actual process. Repeatability measures variation when one operator measures the same item multiple times; reproducibility measures variation when different operators measure the same item. A capable measurement system should contribute less than 10% of total observed variation. Gage R&R is essential in the Measure phase of DMAIC—without good measurements, analysis and improvement efforts are built on unreliable data.
Examples
Before a Six Sigma project on dimensional variation, the team conducted a Gage R&R with 3 operators measuring 10 parts 3 times each. Results showed 35% of variation came from measurement—mostly reproducibility (different operators got different results). They standardized measurement technique before proceeding.
Key Points
- Target: measurement system contributes <10% of total variation
- Repeatability = within-operator variation (same person, same item, multiple measurements)
- Reproducibility = between-operator variation (different people measuring same item)
- Conduct Gage R&R before using data for process analysis
Common Misconceptions
Good equipment means good measurements. Measurement variation often comes from operators, methods, and environment—not equipment. Gage R&R separates these sources. Expensive equipment with inconsistent technique still yields poor data.
Gage R&R is only for physical measurements. Any assessment system benefits from Gage R&R—visual inspections, subjective ratings, software testing. If humans judge outcomes, reproducibility studies reveal inconsistency that undermines decisions.