Abstract

The capability for each separate stream of a process having multiple process streams can be calculated by applying conventional capability metrics such as Cp or Cpk. Because the streams typically have dissimilar averages and standard deviations, the Cpk index will be different for each stream. However, customers will receive shipments that consist of the combined output of all process streams. What Cpk index should be reported to these customers that accurately reflects the quality of this mixture? This article presents a new method for determining a composite capability metric that measures the overall ability of the combined output of several process streams to meet customer requirements. This new metric, called average Cpk, is applicable to processes generating variable data, with either Normal or non-Normal output distributions, as well as those producing attribute data. There is no limit to the number of streams that may be combined and the streams may even have unequal production quantities. Several examples are provided of how the average Cpk metric is applied.