CUSUM control charts with variable sample sizes and sampling intervals

Traditional control charts for process monitoring are based on taking sampl es of fixed size from the process using a fixed sampling interval. Variable sample size (VSS) and variable sampling interval (VSI) control charts vary the sampling rate from the process as a function of the data from the proc ess. By sampling at a higher rate when there is an indication of a change i n the process, VSS and VSI control charts can detect process changes faster than traditional control charts. Previous research has considered the prop erties of CUSUM charts which use the VSI feature. This paper considers CUSU M charts with the VSS feature and with both the VSS and VSI features. Two w ays of developing the control statistic of these charts are considered. It is shown that using either the VSS or VSI feature in a CUSUM control chart will improve the ability to detect all but very large process shifts. The V SI feature usually gives more improvement in detection ability than the VSS feature, but using both features together will give more improvement than either one separately. Guidelines are given for choosing the possible sampl e sizes and the possible sampling intervals for these charts. Methods for s etting up these charts for practical applications are also given.