Parameter estimation and design considerations in prospective applications of the X¯ chart

Abstract

The effects of parameter estimation on the in-control performance of the Shewhart X¯ chart are studied in prospective (phase 2 or stage 2) applications via a thorough examination of the attained false alarm rate (AFAR), the conditional false alarm rate (CFAR), the conditional and the unconditional run-length distributions, some run-length characteristics such as the ARL, the conditional ARL (CARL), some selected percentiles including the median, and cumulative run-length probabilities. The examination involves both numerical evaluations and graphical displays. The effects of parameter estimation need to be accounted for in designing the chart. To this end, as an application of the exact formulations, chart constants are provided for a specified in-control average run-length of 370 and 500 for a number of subgroups and subgroup sizes. These will be useful in the implementation of the X¯ chart in practice.

Keywords:

• Shewhart chart for the mean
• run-length
• false alarm rate
• conditional distribution
• phase 1
• phase 2
• average run-length
• median run-length
• chart constant

Acknowledgement

This work was supported in part by a summer research grant from the University of Alabama.