| Abstract: | The standard deviation chart (S chart) is used to monitor process variability. This paper proposes an upper‐sided improved variable sample size and sampling interval (VSSIt) S chart by improving the existing upper‐sided variable sample size and sampling interval (VSSI) S chart through the inclusion of an additional sampling interval. The optimal designs of the VSSIt S chart together with the competing charts under consideration, such as the VSSI S and exponentially weighted moving average (EWMA) S charts, by minimizing the out‐of‐control average time to signal (ATS1) and expected average time to signal (EATS1) criteria, are performed using the MATLAB programs. The performances of the standard S, VSSI S, EWMA S, and VSSIt S charts are compared, in terms of the ATS1 and EATS1 criteria, where the results show that the VSSIt S chart surpasses the other charts in detecting moderate and large shifts, while the EWMA S is the best performing chart in detecting small shifts. An illustrative example is given to explain the implementation of the VSSIt S chart. |
