THE PROBLEM OF MISPLACED CONTROL LIMITS

The paper draws on simple data analysis and time series ideas to assess the limitations of standard control chart procedures in various practical applications in which the underlying assumptions of control charts may be materially violated and, in consequence, the control limits may be misplaced. To give an idea of the kinds and frequency of violations that occur in practice, we summarize an empirical study of a sample of 235 quality control applications. This collection includes two data sets reported by leading statisticians that are explicitly discussed here to illustrate difficulties in the proper placement of control limits. The results of the study of all 235 data sets suggest that violations of assumptions are the rule rather than the exception in practice.