Sigma metric is more correlated with analytical imprecision than bias

Routine laboratory performance can be assessed by external quality assurance (EQA) programs. The relative bias of the laboratory is assessed against a target value, which may be defined by a higher order method or peer-mean/median. The impression of the participating laboratory may also be estimated if enough replicate measurements are made over the assessment cycle. Sigma metric can be used as an indicator of analytical performance of a laboratory method. This study provided statistical insight into the difference influence of bias and imprecision with the latter having stronger correlation with the sigma metric.