Sigma metric (laboratory method performance)
Computes the sigma metric of a laboratory method.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Total allowable error, the goal
- 10 %
- Bias, signed; it enters as its size
- 2 %
- Imprecision
- 1.5 CV %
Result: sigma in the excellent band, against a 10% allowable error
The tool opens with these values already filled in. Replace them with your own.
What the result means
- The formula
- Total allowable error minus the absolute bias, divided by the imprecision, all as percentages.
- 6 and above
- World class.
- 5 to 6
- Excellent.
- 4 to 5
- Good. 3 to 4 is marginal: workable, but the control rules have to do more of the work.
- Below 3
- Unacceptable. Three is the floor at which the standard control rules can be run at all.
- The goal
- Sigma belongs to a method and a goal together. CLIA, the biological-variation goals, RCPA and EFLM publish different allowable errors for the same analyte.
What you enter
- Total allowable error, the goal
- Bias, signed; it enters as its size
- Imprecision
How this is calculated
Westgard JO, Westgard SA. The quality of laboratory testing today: an assessment of sigma metrics. Am J Clin Pathol. 2006;125(3):343-354. Westgard JO. Six Sigma Quality Design and Control. 2nd ed. Madison: Westgard QC; 2006. Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.