GIFSQ Tools · Module 15 — Metrology, Validation & Laboratory Accreditation · Tool 292 of 500
Score a proficiency testing (PT) round: the z-score against the assigned value, the percentage bias, and the ISO 13528 performance verdict.
Try sample values — fills the form with the worked example below.
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z-score
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Bias (%)
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Verdict code
Equation: z = (x − X) ÷ σₚₜ. Bias (%) = (x − X) ÷ X × 100. |z| ≤ 2 satisfactory; 2 < |z| < 3 questionable; |z| ≥ 3 unsatisfactory.
Method: Proficiency testing scoring per ISO 13528 (statistical methods for use in proficiency testing) and ISO/IEC 17043: the z-score measures the laboratory deviation in units of the proficiency-assessment standard deviation.
Assumptions: the assigned value and σₚₜ come from the PT provider; the result is a single reported value for the round.
Limitations: a satisfactory z-score does not prove trueness if the assigned value itself is biased; a questionable score needs investigation, not a repeat of the same run.
Rounding: z-score to 2 decimal places; bias to 2 decimal places; verdict code 0 satisfactory, 1 questionable, 2 unsatisfactory; intermediates unrounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: Proficiency testing scoring per ISO 13528 (statistical methods for use in proficiency testing) and ISO/IEC 17043: the z-score measures the laboratory deviation in units of the proficiency-assessment standard deviation.
Assumptions: the assigned value and σₚₜ come from the PT provider; the result is a single reported value for the round.
Limitations: a satisfactory z-score does not prove trueness if the assigned value itself is biased; a questionable score needs investigation, not a repeat of the same run.
Rounding: z-score to 2 decimal places; bias to 2 decimal places; verdict code 0 satisfactory, 1 questionable, 2 unsatisfactory; intermediates unrounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: Result 10.6 mg/kg, assigned value 10.0 mg/kg, σₚₜ 0.25 mg/kg: z = (10.6 − 10.0) ÷ 0.25 = 2.40, bias = 6.00% — questionable (verdict code 1).
Enter values above to see results.
Verdict codes: 0 satisfactory (|z| ≤ 2), 1 questionable (2 < |z| < 3), 2 unsatisfactory (|z| ≥ 3). Tool 292 of 500 — GIFSQ
