Type a Uncertainty
Definition
Uncertainty evaluated by statistical analysis of a series of observations. The standard uncertainty is the standard deviation of the mean (standard error) of the repeated measurements. Requires a sufficient number of replicates to be reliable.
- Evaluation method
- Statistical analysis of repeated observations
- Reported as
- Standard deviation of the mean (standard error)
- Requirement
- Sufficient number of replicate measurements
- Pairs with
- Type B uncertainty in combined uncertainty budget
Common questions
Why can Type A uncertainty be unreliable with few replicates?+
The standard error estimate depends on sample size; with only two or three measurements the estimate itself is imprecise, so laboratories set a minimum replicate count before treating the resulting figure as a defensible statistical uncertainty component.
How is Type A uncertainty combined with other uncertainty sources in a reported result?+
It is converted to a standard uncertainty and combined in quadrature (root sum of squares) with Type B components, then multiplied by a coverage factor to produce the expanded uncertainty stated on the certificate or report.
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...
- GUM
- Guide to the Expression of Uncertainty in Measurement. The international standard, maintained by eight metrology and standardisation bodies, that defines Type A...
- 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...