A COMPARISON OF PHASE I CONTROL CHARTS

A carefully done Phase I analysis is a vital part of an overall statistical process control and monitoring regime. Distribution-free control charts can play a useful role in this analysis, as a parametric model assumption often cannot be adequately verified. The performance of two distribution-free charts for the location - the mean-rank and the median chart - are compared in this paper. For benchmarking reasons, the parametric X-chart is included in the comparisons. It is seen that the distribution-free charts are in-control robust, whereas the X-chart is not, and they both perform similarly to the X-chart for normally distributed data. However, for non-normal data, they both outperform the X-chart. The results provide evidence in favour of using distribution-free Phase I charts in practice. Concluding remarks and suggestions for future research are given.