Capability and control

Abstract

In Chapter 6 the emphasis was on the steps to be covered in setting up the initial control charts in order to obtain a state of statistical control. Whether or not the items being processed were within specification was not discussed. It was control that was considered, not capability. The main message was that SPC requires the three stages of data collection, control and improvement. Specification limits, however, cannot be ignored in the short term in the practicalities of industrial life. It is a requirement to have capable processes as well as ones which are under control. At some stage in the sequence indicated in Fig. 7.1 the process will move from an incapable one to a capable one — and the earlier the better.