Nonparametric adaptive EWMA-type control chart with variable sampling intervals

A nonparametric process monitoring scheme with Variable Sampling Intervals (VSI) is investigated to monitor the median in this paper. The proposed scheme is based on the sign test statistic and used a new adaptive-type exponentially weighted moving average control chart. A discrete-state Markov Chain method is presented, and the exact Average Time to Signal (ATS) properties are discussed. The comparative results indicate that the proposed chart can reduce the time in detecting shifts than the corresponding schemes for the fixed sampling interval and provide higher sensitivity for detecting shifts with various magnitudes. The proposed chart's in-control robustness is demonstrated for different continuous distributions using numerical results based on the Monte-Carlo simulation. The implementation of this VSI nonparametric chart is also explained through an illustrative example.