A New Nonparametric Quality Control Technique

Traditional control chart procedures process the information in the time sequence plot of successive values of the control variable. In this paper we consider a control chart procedure that uses the sequential ranks of the observations. The sequential rank of an observation is defined as its rank among the most recent g observations. A control chart technique that considers exponentially weighted moving averages (EWMA) of sequential ranks is proposed. This new nonparametric technique is outlier-resistant, and it performs well if one is concerned about slowly trending process levels.