Guard banding explained with a worked example
A guard band narrows the acceptance zone by a multiple of the uncertainty so that a 'pass' has a controlled probability of being wrong. Here is the arithmetic.
Tolerance ±0.5 °C. Measurement uncertainty U = 0.12 °C (k = 2). With a guard band w = U, acceptance limits become ±0.38 °C. A measured error of 0.40 °C is within tolerance but outside the acceptance limits: under this rule it is not declared a pass.
Why 2.5 %
At a measured value exactly on the acceptance limit (0.38 °C), the true value has a roughly normal distribution centred there with standard deviation U/2 = 0.06 °C. The tolerance limit (0.50 °C) is two standard deviations away, so the probability that the true value exceeds it is about 2.5 %. Inside the acceptance zone the probability is smaller.
Choosing w
- w = U: conservative, common in regulated environments.
- w = 0.5 U or another fraction: when false rejects are costly and the consequence of a false accept is modest.
- w > U: when a false accept is dangerous.
Frequently asked questions
- Does guard banding apply to the laboratory or the customer?
- Either can apply it. The laboratory applies it when making a conformity statement; a customer can apply their own to a reported value and uncertainty.
References
- [1]ILAC-G8: Guidelines on Decision Rules and Statements of Conformity
- [2]JCGM 106:2012 — The role of measurement uncertainty in conformity assessment
General technical guidance written against the cited sources. It is not regulatory or legal advice and does not replace the applicable standard, guideline or a qualified reviewer's judgement.
Have a question on this topic?
Ask ValiTrac AI and see the evidence and calculation behind the answer.
Ask ValiTrac AIFree calculator: Uncertainty budget
Combine Type A and Type B components per the GUM: divisors by distribution, sensitivity coefficients, u_c, effective degrees of freedom and U at k = 2.
Open the calculatorMetrology on ValiTrac
Measurement science, traceability, uncertainty and technical evidence.
Ask a metrology questionRelated articles
What is a decision rule and why every certificate needs one
A decision rule states how measurement uncertainty is accounted for when declaring pass or fail against a tolerance. Simple acceptance, guard banding and non-binary statements are the common choices.
Tolerance versus uncertainty
Tolerance is what you require of the instrument; uncertainty is how well the calibration could measure it. A conformity statement needs both.