Standards profile: ANSI/NCSL Z540.3 and the 2 % false-accept rule

Z540.3 is the US national standard for calibration programs, and its most influential sentence is a probability: where calibrations include conformity decisions, the probability of accepting an out-of-tolerance instrument shall not exceed 2 %.

That one requirement moved American calibration practice from fixed test-uncertainty-ratio rules of thumb toward measurement decision risk. A 4:1 TUR often satisfies the 2 % requirement — but not always, and crucially, a poorer ratio can also satisfy it when the population of instruments rarely approaches its tolerance limits. The standard forces the question 'what is the actual risk?' rather than 'what is the ratio?'.

The mathematics behind that question is JCGM 106's framework of consumer and producer risk, and the practical menu of guard-banded decision rules is ILAC-G8. Together with Z540.3 they form the modern conformity-decision toolkit — all three are profiled in the standards index with links to their official sources.