A distribution-free phase-II CUSUM procedure for monitoring service quality

A new single distribution-free Phase-II CUSUM procedure based on the Cucconi statistic for simultaneously monitoring shifts in the unknown location and scale parameters of a process is proposed. The procedure does not require the assumption of normal data. Control limits are tabulated for practical implementation of the procedure. The in-control and out-of-control performance of the procedure are comprehensively examined in terms of mean, median, variability and some percentiles of the corresponding run length distribution. A thorough comparison with the Shewhart-type procedures based on the Cucconi and Lepage statistics as well as with the CUSUM procedure based on the Lepage statistic is presented. The proposed procedure is illustrated by analysing the service quality of the Vancouver City Call Centre.