PFA stands for probability of false accept: the likelihood that an item actually outside its tolerance is judged in tolerance because of measurement uncertainty. It depends on the TUR, the in-tolerance probability of the population and the decision rule. Some quality programs cap it at 2 % and fall back on a TUR of 4:1 or better where the probability cannot practically be calculated. False accepts are the expensive kind, since bad instruments go back out to measure product.
PFA at that TUR is about 3 percent, over our 2 percent limit, so guard band the acceptance limits.
Also heard as
- probability of false accept
- consumer's risk
- false accept risk
Do not mix it up with
- PFR: PFR is the opposite error: a good item failed because of measurement uncertainty.
Related terms
PFR
Probability of false reject, the chance that an item actually in tolerance is measured and failed.
TUR
Test uncertainty ratio, the tolerance being checked divided by the expanded uncertainty of the measurement process.
Guard band
A margin that pulls acceptance limits inside the tolerance limits to cut the risk of passing nonconforming items.
Specific risk
The probability that one particular measured item is out of tolerance, given its actual measured value.
Decision rule
The agreed method for factoring measurement uncertainty into a pass or fail call against a spec.