The precision-to-tolerance ratio is 6 times the measurement system standard deviation divided by (USL minus LSL); some procedures use 5.15 sigma, which covers 99 percent of gauge spread. Under 0.1 means the gauge eats a tenth or less of the tolerance and is good for product decisions; 0.1 to 0.3 is marginal; above 0.3 the gauge will misclassify a real share of parts near the limits. It judges a gauge for accept or reject decisions, while %GRR against total variation judges it for process control.
P/T is 0.42 on the overlay tool for the new layer. The tolerance got tighter, the gauge didn't.
Also heard as
- precision-to-tolerance ratio
- P/T
- percent tolerance
Related terms
Gauge R&R
Study that measures how much observed variation comes from the measurement system's repeatability and reproducibility.
Ten-to-one rule
Rule of thumb that a gauge should resolve one tenth or less of the tolerance or process variation it measures.
Specification limits
Acceptance boundaries for a product characteristic set by design or customer requirements, labeled USL and LSL.
MSA
Measurement systems analysis: the set of studies that prove a gauge is fit for the decisions its data will drive.