GIFSQ Tools · Module 15 — Metrology, Validation & Laboratory Accreditation · Tool 294 of 500
Trend a control sample against its target and standard deviation: the mean bias, how many results fall outside the ±2 SD warning limits and how many are outside the ±3 SD action limits.
Try sample values — fills the form with the worked example below.
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Mean
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Bias from target
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Outside ±2 SD
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Outside ±3 SD
Equation: Mean = Σx ÷ n. Bias = mean − target. Warning count = results with |x − target| > 2 × SD. Out-of-control count = results with |x − target| > 3 × SD.
Method: Control chart trending per ISO 7870-2 (Shewhart control charts) and ISO/IEC 17025 clause 7.7 (ensuring the validity of results): a control sample is charted against its assigned value, with warning limits at ±2 SD and action limits at ±3 SD.
Assumptions: the target and SD come from the control material certificate or from a stable history; the results are in run order and independent.
Limitations: counts alone do not catch a slow drift or a run of results on one side of the target — use the Westgard Rule Assistant (Tool 295) for pattern rules.
Rounding: mean and bias to 2 decimal places; counts are whole numbers; intermediates unrounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: Control chart trending per ISO 7870-2 (Shewhart control charts) and ISO/IEC 17025 clause 7.7 (ensuring the validity of results): a control sample is charted against its assigned value, with warning limits at ±2 SD and action limits at ±3 SD.
Assumptions: the target and SD come from the control material certificate or from a stable history; the results are in run order and independent.
Limitations: counts alone do not catch a slow drift or a run of results on one side of the target — use the Westgard Rule Assistant (Tool 295) for pattern rules.
Rounding: mean and bias to 2 decimal places; counts are whole numbers; intermediates unrounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: Target 10.0 mg/kg, SD 0.2 mg/kg, results 10.1, 10.3, 9.9, 10.5, 10.2, 9.8, 10.0, 10.4: mean = 10.15, bias = 0.15 mg/kg, outside ±2 SD = 1 (10.5), outside ±3 SD = 0.
Enter values above to see results.
One result outside ±3 SD stops the run; two outside ±2 SD on the same side also stops it — see Tool 295 for the full rule set. Tool 294 of 500 — GIFSQ
