Lower confidence limits for critical systematic errors

Objectives: The critical systematic error (ΔSEC) can be used to select a quality control scheme. It is just an estimate of the true ΔSEC, however, and its precision depends on the sample size. Thus, the lower confidence limit (LCL) of ΔSEC should be adopted instead to guard against false optimism. Design and methods: LCLs are calculated as a function of ΔSEC and sample size using a modified version of Bissellʹs formula. Results: The uncertainty in ΔSEC may be significant. The largest relative difference between ΔSEC and its LCL is 52.2%, for ΔSEC = 2 and n = 20. This difference decreases with increasing ΔSEC and sample size. Conclusions: It is shown that taking into account the uncertainty in ΔSEC 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.