GIFSQ Tools · Module 4 — Microbiology Methods & Laboratory Operations · Tool 80 of 500
Score a proficiency-testing result as a z-score against the assigned value and the standard deviation for proficiency assessment, per ISO 13528.
Try sample values — fills the form with the worked example below.
Proficiency log
Saved on this device only. Calculate a result first, then add it to the log.
| Date / time | Result | Assigned | SD | z-score | Assessment |
|---|
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z-score
–
|z|
–
Assessment
Equation: z = (laboratory result − assigned value) ÷ standard deviation for proficiency assessment. Satisfactory when |z| ≤ 2. Questionable when 2 < |z| < 3. Unsatisfactory when |z| ≥ 3.
Method: ISO 13528 (statistical methods for use in proficiency testing by interlaboratory comparison), z-score assessment of a laboratory's result against the assigned value.
Assumptions: the assigned value and the standard deviation for proficiency assessment come from the proficiency-testing provider; the result and the assigned value are in the same units.
Limitations: a single satisfactory z-score does not prove a method is under control — track z-scores over rounds for drift and bias. A questionable or unsatisfactory score needs investigation, not a repeat of the same result.
Rounding: z-scores to 2 decimal places. The assessment bands are applied to the unrounded z-score.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: ISO 13528 (statistical methods for use in proficiency testing by interlaboratory comparison), z-score assessment of a laboratory's result against the assigned value.
Assumptions: the assigned value and the standard deviation for proficiency assessment come from the proficiency-testing provider; the result and the assigned value are in the same units.
Limitations: a single satisfactory z-score does not prove a method is under control — track z-scores over rounds for drift and bias. A questionable or unsatisfactory score needs investigation, not a repeat of the same result.
Rounding: z-scores to 2 decimal places. The assessment bands are applied to the unrounded z-score.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: Laboratory result 105, assigned value 100, standard deviation for proficiency assessment 2. z = (105 − 100) ÷ 2 = 2.50; |z| = 2.50, which is between 2 and 3, so the result is Questionable.
Enter values above to see results.
Log every proficiency round, satisfactory or not. A run of z-scores on one side of zero is a bias signal even when every score is satisfactory. Tool 80 of 500 — GIFSQ
