6.1 Measurement System Analysis (MSA / Gauge R&R)

Before trusting data, you must trust the tool that generated it. If your ruler is elastic, every measurement is a lie. Measurement System Analysis (MSA) quantifies the error introduced by the gauge and the operator. It answers the critical question: "Is the variation I see real, or is it just noise in my test equipment?" If your measurement error exceeds 30%, you are effectively flipping a coin to determine Pass/Fail.

Gauge R&R (Repeatability & Reproducibility)

We decompose measurement error into two distinct vectors. You must isolate them to fix them.

Repeatability (Equipment Variation):

The inherent precision of the hardware.

• Test: One operator measures the same part 10 times.
• If variation is high → Then the fixture is loose, the sensor is noisy, or the part is not seated consistently.

Reproducibility (Appraiser Variation):

The consistency between human operators.

• Test: Three different operators measure the same part.
• If variation is high → Then the SOP is ambiguous, or the reading method is subjective (e.g., "visual estimation").

Pro-Tip: Always perform Gauge R&R using parts that span the full tolerance range (Low, Nominal, High). If you only measure "perfect" parts, you will calculate a false sense of security.

The Logic of Acceptance (AIAG Standard)

Do not debate these numbers. They are the industry standard (AIAG MSA Manual).

Error Band Logic:

• If Gauge R&R (GRR) < 10% → Then System is Capable. No action required.
• If GRR is 10% – 30% → Then System is Marginal.
  • Action: Acceptable only for non-critical dimensions OR if the cost of a better gauge is prohibitive. Requires Quality Manager sign-off.
• If GRR > 30% → Then System is Unacceptable.
  • Action: Stop Use. Any data collected is statistically invalid. Repair the fixture or retrain operators immediately.

Number of Distinct Categories (ndc)

Precision is useless without resolution. The ndc metric tells you how many "buckets" your ruler has within the process range.

Resolution Rule:

• If ndc < 5 → Then the gauge is too dull to detect process changes. It is like measuring a hair's width with a yardstick.
• Action: Switch to a gauge with higher resolution (e.g., move from calipers to micrometers).

Final Checklist