Objectives: The critical systematic error (DeltaSE(C)) can be used to select a quality control scheme. It is just an estimate of the true DeltaSE(C), however, and its precision depends on the sample size. Thus, the lower confidence limit (LCL) of DeltaSE(C) should be adopted instead to guard against false optimism.
Design and methods: LCLs are calculated as a function of DeltaSE(C) and sample size using a modified version of Bissell's formula.
Results: The uncertainty in DeltaSE(C) may be significant. The largest relative difference between DeltaSE(C) and its LCL is 52.2%, for DeltaSE(C)=2 and n=20. This difference decreases with increasing DeltaSE(C) and sample size.
Conclusions: It is shown that taking into account the uncertainty in DeltaSE(C) can greatly influence quality control planning decisions. Caution should thus be exercised when interpreting control data and choosing an appropriate quality control strategy. The derived equation for LCL is simple, and can be immediately applied to practical solutions.