Measurement System Analysis
Also known as: MSA, Gauge R&R
Definition
Measurement System Analysis (MSA) is a set of statistical methods used to quantify the variation in a measurement system itself — gauge, operator, and method — so that observed process variation can be trusted.
In depth
If your gauge can't reliably distinguish a good part from a bad one, every downstream SPC chart, capability calculation, and pass/fail decision is suspect. MSA separates total observed variation into 'part-to-part' (real process variation) and 'measurement system' (gauge + operator + method noise).
The most common MSA study is Gauge R&R (Repeatability & Reproducibility): multiple operators measure multiple parts multiple times, and ANOVA partitions the variance. The AIAG MSA manual recommends measurement-system variation be <10% of total variation (acceptable), and flags <30% as marginal.
MSA is one of the six automotive Quality Core Tools and is required at PPAP. Treating it as a one-time PPAP checkbox is the most common audit finding — gauges drift, and MSA needs periodic re-validation.