The between-run component of variation in internal quality control.

Design of control charts for the mean, the within-run component of variance, and the ratio of between-run to within-run components of variance is outlined. The between-run component of variation is the main source of imprecision for analytes determined by an enzymo- or radioimmunoassay principle; accordingly, explicit control of this component is especially relevant for these types of analytes. Power curves for typical situations are presented. I also show that a between-run component of variation puts an upper limit on the achievable power towards systematic errors. Therefore, when the between-run component of variation exceeds the within-run component, use of no more than about four controls per run is reasonable at a given concentration.