Abstract

The information on auxiliary characteristics helps significantly in increasing the efficiency of control charts for detecting shifts in process parameters. In this study we proposed Shewhart type control charts, namely the  chart, the  chart and the  chart, which utilizes information on two auxiliary characteristics (X and Z) for improved monitoring of process location parameter with respect to a single quality characteristic of interest (Y). Assuming trivariate normality of (Y, X, Z), a general control chart structure is developed in the form of the three sigma and the probability limits. The performance of the proposed charts is compared with the usual Shewhart  chart, the  chart of Riaz (2008a), the control limits of Zhang (1984) and Wade and Woodall (1993). It has been observed that the proposed charts perform superior, in terms of discriminatory power, as compared to the above mentioned counterparts, depending upon the correlation structure among the auxiliary characteristics and the quality characteristic of interest. The said superiority zone of the correlation structures, favoring the proposed charts, needs to be identified very carefully to apply it in a given situation.

Key words: Auxiliary characteristics mean control charts, location parameter, normality, power curves, quality characteristics.