Critical Comparison of Moving Average and Cumulative Summation Control Charts for Trace Analysis Data

摘要

Percentage recovery estimates have been obtained for 15 analytes or surrogates of environmental concern by four commercial laboratories over a two-year period. These quality control analyses were performed using standardized methods on a control soil matrix. Reproducibility of recovery estimates was surprisingly consistent from lab-to-lab, which permitted calculation of performance (Capability) limits based on the mean recovery + or -3 sigma. From a comparison of moving average control charts (N = 2 and N = 3) with cumulative summation charts, the N = 3 moving average charts were considered most suitable for routine lot-to-lot control by contractors. The cumulative summation charts are very useful for situations requiring critical diagnostic analysis of problems. Where duplicate recoveries were obtained with each lot-to-lot variability was similar in magnitude to within-lot variability. To avoid an excessive number of out-of-control responses, control limits should be based on total variability rather than within-lot variability.