Measurement Systems Analysis (Gage Studies)

For measurement systems that result in quantitative measurements such as weight, concentration, or strength, it is important to determine the magnitude of any error in the resulting measurements. If the error is large, it may be impossible to determine whether or not an individual sample is within spec. In addition, designed experiments rely on the ability to separate real effects of making changes from the background noise and could be sabotaged by an inadequate measurement system.

While repeatability and reproducibility studies concentrate on the variability or precision of a measurement system, Linearity and Accuracy studies quantify the bias. In these studies, multiple measurements are made on reference samples for use with measurement systems analysis software. Then an equation is constructed for the bias of the measurements.

When the results of a measurement system are PASS or FAIL rather than a quantitative value, special procedures are necessary for measurement systems analysis. STATGRAPHICS Centurion systems analysis software provides three procedures outlined by the AIAG to deal with such systems: the risk analysis method, the signal theory method, and the analytic method. In the risk analysis method, multiple appraisers measure samples with known characteristics. Statistics are calculated based on how often the appraisers correctly characterize each sample and how frequently they agree with themselves and each other.

The GLM Method estimates the repeatability and reproducibility of a measurement system based on a study in which m appraisers measure n items r times. It also estimates important quantities such as the total variation, the precision-to-tolerance ratio, the standard deviation of the measurement error, and the percent of study contribution from various error components. In addition to variation introduced by appraisers and parts, additional factors may also be included. The additional factors may be treated as having either fixed or random effects. Unlike the other Gage R&R  procedures, this procedure can deal with unbalanced data.

More: Gage Studies - GLM Method.pdf or Watch Video