CUSUM Charts for Dispersion Matrix with Variable Sampling Intervals

Cumulative sum(CUSUM) control charts for monitoring dispersion matrix of related quality characteristics with variable sampling intervals(VSI) are investigated. Properties of the proposed charts are evaluated for matched fixed sampling interval(FSI) and VSI charts in terms of average time to signal(ATS) and average number of samples to signal(ANSS). Shewhart charts are also proposed to compare the performances of the CUSUM charts. The numerical results show that CUSUM quality control scheme based on the sample statistic TV_i is preferable and VSI CUSUM chart with small reference value is more efficient for small or moderate shifts.