Definitions

Capability of Assay1	Assays are classified as capable based on the sigma metric:
	Sigma	Capability
	Less than 3	Incapable
	Greater than or equal to3	Capable
	Greater than or equal to 6	Highly capable
Six Sigma2	The number of standard deviations (SD) spanning the product specifications on the defect rate and defects per million
	SD range	Defect Rate (%)	Defects/Million
	± 2 SD	4.5	45,400
	± 3 SD	< 0.27	<2,700
	± 4 SD	0.0063	63
	± 5 SD	0.0057	0.57
	± 6 SD	0.000002	0.002
Sigma Metric or Assay Capability (Cps)1,2	$$\mathbf{\text{Sigma}}\hspace{0.17em}\mathbf{\text{Metric}}=\frac{\%\mathbf{\text{TEA}}-\%\mathbf{\text{Bias}}}{\%\mathbf{\text{CV}}}$$ Where %TEA = Total Error Allowable % ○ Source = Biological Variation Goals (Optimal, Desirable, Minimal) RCPAQAP Allowable Limits of Performance3 %Bias = Method Bias vs. True Value (Target Value) %CV = Method imprecision
Systematic Error (4)	Due to identified causes and can, in principle, be eliminated. Errors of this type result in measured values that are consistently too high or consistently too low. Types – Instrumental – Observational e.g. parallax in reading a meter scale – Environmental – Theoretical - due to simplification of the model system or approximations in the equations describing it.
Random Error4	Random errors are positive and negative fluctuations that cause about one-half of the measurements to be too high and one-half to be too low. – Sources of random errors cannot always be identified. Types of random error – Observational e.g. errors in judgment of an observer when reading the scale of a measuring device to the smallest division. – Environmental e.g. unpredictable fluctuations in line voltage, temperature, or mechanical vibrations of equipment.
Structure of QC Rules	QC rules of the form µi ± k*SDi Where µi is the target mean, k is a variable and SD is the target SD. Are centered on the instrument’s mean concentrations Limits are based on multiples of the instrument’s SD Multiple instruments measuring the same analyte have different rejection limits QC rules of the form µu ± f*TEA Where f is some proportion (%) and TEA is the total error allowable or allowable limit of performance. Are centered on the unbiased QC concentrations Limits are based on a fraction of the total allowable error Multiple instruments measuring the same analyte have the same rejection limits
Westgard Rules5	QC rules often used for evaluation of QC data.