Properties of VSI EWMA Charts for Monitoring Correlation Coefficients of Related Quality Characteristics

Properties of the variable sampling interval(VSI) exponentially weighted moving average(EWMA) control charts for monitoring correlation coefficients of related quality characteristics are investigated. To compare the efficiencies of the proposed charts, the performances 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). Average number of swiches(ANSW) of the proposed VSI charts are also investigated. From the numerical computation, we found that small values of smoothing constant with accumulate-combine approach is more effective in detecting small or moderate shifts in EWMA charts.