Most proficiency testing schemes turn your result into a performance score. The most common is the z-score: the difference between your result and the assigned value, divided by the standard deviation used for proficiency assessment.
The usual rule of thumb
- |z| of 2 or less: satisfactory.
- |z| between 2 and 3: questionable, a warning signal.
- |z| of 3 or more: unsatisfactory, an action signal.
Always check the criteria in your scheme's own report, because some schemes use different limits or other scoring methods.
Look beyond a single round
One acceptable score tells you little. Look at the pattern across rounds. Several results that all sit on the same side of the assigned value point to bias, even when every individual score looks fine. Rolling averages make that visible early, before the cost of correcting it grows.
When a result is flagged, treat it as a prompt for root cause analysis: method, calibration, sample handling, calculation or transcription. Document what you find and what you changed, because assessors will ask.
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