Z-Score (Proficiency Testing)
Definition
A performance statistic in PT defined as z = (x - X) / s_PT, where x is the laboratory's result, X is the assigned value, and s_PT is the standard deviation for proficiency assessment. |z| ≤ 2 is satisfactory, 2 < |z| < 3 is a warning, and |z| ≥ 3 is unsatisfactory under ISO 13528.
- Formula
- z = (x - X) / s_PT
- Satisfactory
- |z| <= 2
- Warning
- 2 < |z| < 3
- Unsatisfactory
- |z| >= 3
- Standard
- ISO 13528
Common questions
What does an unsatisfactory z-score mean for a laboratory's accreditation?+
A single unsatisfactory result triggers a documented investigation into the cause, such as method, calibration or operator issues, and repeated or unresolved unsatisfactory scores on the same test can lead an accreditation body to restrict the laboratory's scope for that test until corrective action is verified.
Why is the assigned value X not simply the average of all participating labs?+
Proficiency testing schemes often set the assigned value from a reference method, a formulated or certified sample, or a resistant statistic such as a trimmed mean, precisely so a cluster of labs sharing the same systematic error does not pull the reference value toward their shared mistake.
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 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...