A margin, usually based on the expanded uncertainty, that moves the acceptance limit inward from the tolerance limit, so a reading must fall comfortably inside spec to count as a pass. With a guard band equal to U at k = 2, a reading sitting exactly on the acceptance limit carries only about a 2 % chance of actually being out of tolerance. Guard banding buys lower false-accept risk at the price of more false rejects, rework and adjustment.
With the guard band applied the acceptance limit is plus or minus 0.85, so that 0.9 is a fail even though it's inside tolerance.
Also heard as
- guard banding
- guardband
Do not mix it up with
- Tolerance: Tolerance is the spec limit itself; the guard band shifts the pass line inside it to account for uncertainty.
Related terms
Decision rule
The agreed method for factoring measurement uncertainty into a pass or fail call against a spec.
PFA
Probability of false accept, the chance that a nonconforming item is measured and passed as good.
PFR
Probability of false reject, the chance that an item actually in tolerance is measured and failed.
Tolerance
The allowed deviation from nominal that an item or instrument may show and still be accepted.
Specific risk
The probability that one particular measured item is out of tolerance, given its actual measured value.