Chapter 5

Control Charts

The fact that the criterion which we happen to use has a fine ancestry in highbrow statistical theorems does not justify its use. Such justification must come from empirical evidence that it works. (Shewhart, 1931, p. 18)

Overview

Control charts or process behaviour charts have been used for nearly 90 years to monitor process performance. Although originally developed for use in manufacturing industry they are now widely applied to processes involving the provision of services in fields such as finance and healthcare.

This chapter deals with a wide variety of control charts and with their creation, interpretation and maintenance via Minitab. Variables charts enable the monitoring of continuous random variables (measurements), while attribute charts monitor discrete random variables (counts). The consequences of tampering with processes are illustrated. Reference is made to autocorrelated data and feedback adjustment. Time-weighted control charts will be introduced, as will multivariate charts for the simultaneous monitoring of two or more variables.

The term ‘control charts’ suggests that these tools have a role only in the control phase of Six Sigma projects. However, as Figure 1.4 indicates with reference to the transient ischaemic attack and stroke clinic project, they may also be employed during the measure, analyse and improve phases. Indeed, the team involved with the project resolved that the control charts should continue to be maintained as a ...

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