Control Charts for Correlated Quality Characteristics with Two Sampling Intervals

Numerical performances of the multivariate control charts for simultaneously monitoring means of several correlated quality characteristics are investigated. Properties of the Shewhart and CUSUM charts based on likelihood ratio test (LRT) statistic evaluated for matched fixed sampling interval (FSI) and variable sampling interval (VSI) with two sampling intervals. When occurring a small or moderate shift, we noticed that CUSUM chart has a small average time to signal (ATS) and a small average switching rate (ASWR) when it is compared with Shewart chart. In addition we also noticed that two sampling interval chart with (d₁,d₂) = (0.1, 1.9) was more efficient than the chart with (0.7, 1.3). If two sampling interval charts do not require too much additional cost and effort comparing to FSI procedure, multivariate CUSUM chart with two sampling interval chart can be recommended for small or moderate shift in production process.