Evaluation of measurement uncertainty (clause 7.6)
Laboratories must identify contributions to uncertainty and, for calibration, evaluate the uncertainty of every result — including calibration of their own equipment.
Clause 7.6 requires laboratories to identify the contributions to measurement uncertainty and take all significant contributions into account using appropriate methods of analysis. A laboratory performing calibrations, including of its own equipment, must evaluate the measurement uncertainty for all calibrations.
What 'appropriate methods' means in practice
- The GUM's law of propagation for analytical budgets.
- Monte Carlo (GUM Supplement 1) where the model is non-linear or the distributions are far from normal.
- Documented budgets that can be reproduced and audited.
ILAC-P14 adds the accreditation-body policy: the CMC listed in the scope defines the floor, and the reported uncertainty on a certificate must include the contributions of the customer's device, so it is normally larger than the CMC.
Frequently asked questions
- Can a laboratory report an uncertainty smaller than its CMC?
- No. ILAC-P14 states the reported uncertainty must not be smaller than the CMC for that calibration.
References
- [1]ISO/IEC 17025:2017 — General requirements for the competence of testing and calibration laboratories
- [2]ILAC-P14: ILAC Policy for Measurement Uncertainty in Calibration
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.
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