ISI - IR

Several studies have shown that we can improve the statistical performance of control charts by changing their parameters during the production process. Such charts can be classified, from an implementation point of view, into two categories: charts with parameters that are fixed but not constant for the duration of the monitoring operation, and charts for which at least one of the parameters is allowed to change in real time, taking into account current sample information. The latter are called adaptive charts and the previous are called control charts with predetermined parameters. In this context, several sampling schemes were presented and analyzed. In this paper we present a new method where the sampling instants are schedule at the beginning of the sampling process according to the system lifetime distribution and the samples are taken using the double sampling method. We studied this combined double sampling and predetermined sampling intervals (DSPSI) method using a Shewhart control chart, considering different lifetime distributions in order to evaluate its performance for systems with different hazard rates. The obtained results are analyzed in a critical way and are compared, with the ones obtained for other sampling schemes previously presented, considering AATS (Adjusted Average Time to Signal) and ANI (Average Number of inspected Items) values. Particularly, the new method provides a valuable alternative to adaptive sampling schemes. In general, we can draw the conclusion that this method is more efficient than the others sampling schemes considered, with differences that sometimes are very high, particularly when used in systems with an increasing failure rate distribution