MSA is a set of studies that check whether a measurement system can be trusted before its numbers are used for decisions. The measurement system is more than the gauge: it includes the people, the method, the fixture and the environment. MSA looks at bias (is the average reading right), linearity (is it right across the range), stability (does it stay right over time), and repeatability and reproducibility, usually studied together as a gauge R&R. For attribute checks such as go/no-go gauges, an attribute agreement study is used instead.
Example: Before using a new bore gauge for SPC, a lab measures a reference ring of known size 20 times to check bias, repeats that each week to check stability, and runs a gauge R&R with three operators on ten production parts.
Every capability number, control chart and accept or reject call depends on the measurement. If the gauge varies as much as the parts do, good parts get scrapped and bad parts get shipped, and no amount of process work will fix it.
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