gage repeatability

Gage Repeatability is the variation in measurements when the SAME operator measures the SAME part multiple times with the SAME gage — also called equipment variation (EV), it measures the pure instrument precision, excluding any operator influence.

Repeatability Assessment

• Method: One operator measures the same part 10-30 times — calculate the standard deviation of repeated measurements.
• EV: $EV = \bar{R} / d_2$ (using range method) or directly from ANOVA — the repeatability standard deviation.
• Sources: Gage resolution, mechanical precision, environmental sensitivity (temperature, vibration), and sample positioning.
• Units: Same units as the measured parameter — or as a percentage of tolerance.

Why It Matters

• Baseline: Repeatability is the MINIMUM measurement uncertainty — reproducibility and other factors add more.
• Resolution: If repeatability is poor, the gage may lack sufficient resolution — need a better instrument.
• Semiconductor: CD-SEM repeatability must be <0.5nm for advanced node CD measurements — extremely demanding.

Gage Repeatability is the instrument's consistency — how well the measurement tool reproduces the same result when measuring the same item repeatedly.