SC QUALITY GUILDAccuracy and precision check
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Measure a reference part several times and see whether the gauge is accurate (no bias) and precise (repeatable), with the five MSA terms side by side.
ON THE EXAM · CSSYB III.C.1 · FREE · NOTHING YOU TYPE LEAVES YOUR BROWSER
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Accuracy and precision are different
Accuracy is how close the average reading is to the true value. The difference is the bias. Precision is how close repeat readings are to each other. On a target, accurate and imprecise is a wide spread centered on the bullseye; precise and inaccurate is a tight group off to one side. A gauge needs both.
How the check works
Measure one reference part, with a certified or known value, several times under normal conditions. The tool reports:
- Bias, the average minus the reference, and whether it is statistically significant. A t-test compares the bias with what chance alone would produce given the scatter and the number of readings.
- Repeatability, the standard deviation of the readings. Six times it covers most of the spread; the tool shows it as a percentage of the tolerance.
The 10% and 30% guidelines for the spread come from gauge R&R practice, where under 10% is generally acceptable, 10 to 30% may be acceptable depending on the application, and over 30% is not. This check covers one appraiser and one part, so it measures repeatability only. Use a gauge R&R for the full picture.
The other three terms
Reproducibility is the variation between appraisers. Linearity is whether the bias changes across the range of the gauge. Stability is whether the bias drifts over time. A gauge can be accurate at the middle of its range and biased at the ends, or accurate in January and biased by June.
On the Yellow Belt exam
Measurement system analysis terms are topic III.C.1 of the 2022 CSSYB Body of Knowledge, and gauge R&R is III.C.2. Expect to match a described result to accuracy, precision, bias, linearity or stability.