SOME PROCESS CAPABILITY INDEXES ARE MORE RELIABLE THAN ONE MIGHT THINK

Abstract

In this paper we obtain formulae for exact expected values and standard deviations of estimators of certain process capability indices discussed by Bissell. In particular, we show that for the index C(pk) Bissell's formula gives values for the standard deviation which are too high especially when the actual population mean value is close to (or equal to) the average of the upper and lower specification limits.