GUM
Definition
Guide to the Expression of Uncertainty in Measurement. The international standard, maintained by eight metrology and standardisation bodies, that defines Type A and Type B uncertainty evaluation, uncertainty propagation, and expanded uncertainty. ISO/IEC 17025 requires laboratories to implement GUM-consistent uncertainty procedures.
- Full form
- Guide to the Expression of Uncertainty in Measurement
- Maintained by
- Eight international metrology and standardisation bodies
- Covers
- Type A and Type B uncertainty evaluation, propagation, expanded uncertainty
- Compliance link
- ISO/IEC 17025 requires GUM-consistent procedures
Common questions
What is the practical difference between Type A and Type B uncertainty evaluation?+
Type A uncertainty is derived statistically from repeated measurements, such as the standard deviation of replicate readings. Type B uncertainty is derived from other information, like calibration certificates, manufacturer specifications, or prior experience, when repeated measurement is not practical for that source of error.
Why does a forensic laboratory need to report expanded uncertainty rather than a single value?+
A single measured value without an uncertainty range gives no indication of how confidently it can be relied upon, which matters when a result is close to a legal threshold, such as a blood alcohol limit. Expanded uncertainty gives a coverage interval within which the true value is expected to lie.
Related terms
- Combined Standard Uncertainty
- The overall standard uncertainty of a result, obtained by combining all Type A and Type B component uncertainties in quadrature (square root...
- Expanded Uncertainty
- The combined standard uncertainty multiplied by a coverage factor k, chosen to achieve a stated coverage probability. For a normal distribution, k...
- Type a Uncertainty
- Uncertainty evaluated by statistical analysis of a series of observations. The standard uncertainty is the standard deviation of the mean (standard error)...
- Type B Uncertainty
- Uncertainty evaluated by means other than statistical analysis of repeated measurements. Sources include calibration certificates, instrument specifications, reference data, and expert scientific...
- Z-Score (Proficiency Testing)
- A performance statistic in PT defined as z = (x - X) / s_PT, where x is the laboratory's result, X is...