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

Nonparametric control chart techniques represent a rather neglected area in Sta-tistical Process Control, in particular with respect to monitoring process variability. There are only very few relevant papers published. The reason for not regarding nonparametric tech-niques 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.