Chapter 8Control Charts for Count Processes
8.1 Introduction to Statistical Process Control
Methods of statistical process control (SPC) help to monitor and improve processes in manufacturing and service industries, and they are also often used in fields such as public-health surveillance. For the given process, relevant quality characteristics are measured over time, thus leading to a (possibly multivariate) stochastic process
of continuous-valued or discrete-valued random variables (variables data or attributes data, respectively). Examples of such quality characteristics could be the diameter of a drill hole (variables data) or the number of non-conformities (attributes data) in a produced item, or the number of infections in a health-related example. One of the most important SPC tools is the control chart, which requires the relevant quality characteristics to be measured online. Control charts are applied to a process operating in a stable state (in control); that is,
is assumed to be stationary according to a specified model (the in-control model). As a new measurement arrives, it is used to compute a statistic (possibly also incorporating past values of the quality characteristic), which is then plotted on the control chart with its control limits. If the statistic violates ...