Control charts and process capability

A new design algorithm for the (X) over bar &S control charts is presented in this article, in which the detection power is allocated between the (X) over bar 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-pk. Specifically, when C-pk decreases to a specified value in any possible pattern, the (X) over bar &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-sigma solution.