A Note on the Efficiency of Nonparametric Control Chart for Monitoring Process Variability

Nonparametric control chart techniques represent a rather neglected area in Statistical Process Control, in particular with respect to monitoring process variability. There are only very few relevant papers published. The reason for not regarding nonparametric techniques for controlling process variability are investigated in this paper by means of a control chart based on the two sample rank-sum test of Ansari and Bradley. Using the popular 3σ-control limits, the in-control state performance (ARL) is compared by simulation with that of a Shewhart S-chart for different distributions and different sample sizes. Moreover, the efficiency to detect shifts in the variability is investigated. It is shown that the in-control performance is excellent, however, the performance in the out-of-control state is rather poor.