Control charts and process capability

A new design algorithm for the X~&S control charts is presented in this article, in which the detection power is allocated between the X~ chart and the S chart in an optimal manner. This algorithm associates the out-of-control conditions of a process with a degraded value of the process capability ratio C/sub pk/. Specifically, when C/sub pk/ decreases to a specified value in any possible pattern, the X~&S control charts will give a signal within a specified time period with a predetermined confidence level. This algorithm allows the quality assurance (QA) engineers to accurately specify the performance characteristics of the control charts (i.e., the in-control and out-of-control run lengths and the associated confidence levels) and assist them to achieve the desired 6-/spl sigma/ solution.