Glossary

The abbreviations that appear on measurement study reports, in plain language. Start typing to filter.

%GRR: Measurement variation as a percentage of either total study variation or of tolerance. Under 10 percent is generally acceptable, 10 to 30 percent is conditionally acceptable, and above 30 percent is not acceptable. Always confirm which denominator was used.
ANOVA method: Analysis of Variance. The preferred method for computing a Gauge R and R because it estimates variance components directly and can also detect part-by-operator interaction.
Attribute agreement analysis: The MSA method for pass / fail, go / no-go, and visual inspection. It measures how often appraisers agree with themselves, with each other, and with a known standard. A variable Gauge R and R does not apply to this kind of data.
AV: Appraiser Variation. The report abbreviation for reproducibility - the variation attributable to differences between operators.
Average and Range method: The older hand-calculable Gauge R and R method using range statistics and lookup constants. Simpler to compute but it cannot detect interaction between parts and operators.
Also known as: X-bar and R method
Bias: A systematic offset between the average measurement and a known reference value. A biased gauge reads consistently high or consistently low, and a Gauge R and R will not detect it.
EV: Equipment Variation. The report abbreviation for repeatability - the variation attributable to the gauge itself.
Gauge R and R: A study that separates measurement variation into repeatability and reproducibility. It is one study within MSA, not the whole of it.
Also known as: GRR, Gage R and R
GRR: The combined measurement variation. Because variances add, GRR is the square root of EV squared plus AV squared - not EV plus AV.
Interaction: When operators disagree on some parts but not others. It signals that the measurement difficulty depends on the part - often a fixturing or feature-access problem rather than a training problem.
Also known as: Part-by-operator interaction
Linearity: Change in bias across the measurement range. A gauge may be accurate near the middle of its range and significantly off near the extremes.
MSA: Measurement System Analysis. The discipline of quantifying how much of the variation in your data comes from the measurement process rather than from the parts. It covers bias, linearity, stability, repeatability, and reproducibility.
Also known as: Measurement System Analysis
ndc: Number of Distinct Categories. How many separate groups the measurement system can reliably tell apart within the part variation. A value of 5 or more is normally required; below 5 the gauge is effectively sorting rather than measuring.
Also known as: Number of distinct categories, discrimination ratio
PV: Part Variation. The genuine part-to-part difference in the sample used for the study. If the sample parts are too similar, PV is small and %GRR looks artificially bad.
Also known as: Part-to-part variation
Repeatability: Scatter observed when the same operator measures the same part with the same gauge more than once. It reflects the gauge and the method, not the people.
Also known as: EV, equipment variation
Reproducibility: Differences between operators measuring the same parts. Usually caused by technique, training, or ambiguous work instructions.
Also known as: AV, appraiser variation
Resolution: The smallest increment the gauge can display. A common rule of thumb is that resolution should be no coarser than one tenth of the tolerance or of the process variation.
Also known as: Discrimination, least count
Stability: Consistency of the measurement system over time. Assessed by charting repeated measurements of a master part across weeks or months.
Also known as: Drift
Tolerance: The permitted range for a characteristic, upper specification limit minus lower specification limit. When %GRR is expressed against tolerance rather than study variation, it answers whether the gauge is fit to judge conformance.
TV: Total Variation for the study. The square root of GRR squared plus PV squared. This is the denominator when %GRR is expressed against study variation.
Also known as: Total study variation

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