Gage R&R

Gage R&R (Gage Repeatability and Reproducibility) is a structured Measurement System Analysis (MSA) method used to quantify how much variation in measured data comes from the measurement system itself, rather than from the parts or process being measured. It evaluates both the measuring device (gage) and the people and methods using it.

What Gage R&R measures

Gage R&R studies typically break measurement system variation into two main components:

• Repeatability: Variation when the same operator measures the same part multiple times with the same instrument under the same conditions.
• Reproducibility: Variation when different operators (or setups, shifts, or locations) measure the same part using the same instrument and method.

The combination of these is compared to the overall part or process variation to determine whether the measurement system is suitable for tasks such as inspection, SPC, capability studies, FAI, or PPAP.

How Gage R&R is used in manufacturing

In industrial and regulated environments, Gage R&R commonly refers to:

• Running a planned study where multiple operators measure a set of representative parts multiple times.
• Statistically analyzing the results (often using ANOVA or range-based methods) to estimate measurement system variation.
• Judging whether the gage and method are acceptable for control, release, or compliance activities based on internal or customer criteria.
• Supporting documented evidence of measurement system suitability for quality systems, audits, FAI (AS9102), PPAP, and ongoing inspection plans.

Results are often expressed as a percentage of total variation (e.g., %GRR of total process variation) or as a percentage of tolerance (e.g., %GRR of the specification range).

What Gage R&R includes and excludes

Gage R&R includes:

• Variation from the measurement device, operator, method, and short-term conditions used in the study.
• Both attribute (pass/fail) and variable (numeric) measurement systems, although the analysis methods differ.

Gage R&R does not by itself:

• Prove that a process is capable or stable; it only evaluates the measurement system.
• Replace instrument calibration or maintenance programs.
• Guarantee compliance to any standard; it provides quantitative evidence used within a quality system.

Common confusion

• Gage R&R vs. MSA: Gage R&R is one subset of Measurement System Analysis. MSA also includes bias, linearity, stability, and attribute agreement studies.
• Gage R&R vs. process capability: Gage R&R evaluates the measurement system; capability indices such as Cpk or Ppk evaluate the manufacturing process using measurement data that should already come from an acceptable measurement system.

Relation to FAI and PPAP

In aerospace and automotive programs, Gage R&R is often required or expected as part of readiness for First Article Inspection (AS9102) or PPAP. A completed and documented Gage R&R study helps show that key inspection results, capability studies, and control plans are based on a capable and understood measurement system.