ABSTRACT
Chapter 6 introduces the use of mean and range charts for the control of process accuracy and precision for variables and provides the method by which process control limits may be calculated. It sets out the steps in assessing process stability and capability and examines the use of mean and range charts in the real-time control of processes.
The control limits on the mean and range charts are based on simple calculations from the data and the interpretation of the plots are based on rules for action, warning and trend signals. Mean and range charts are used together to control the process. A set of rules is required to assess the stability of a process and to establish the state of statistical control.
Mean and range charts may be used to monitor the performance of a process. There are three zones on the charts which are associated with rules for determining what action, if any, is to be taken. SPC for variables is in three stages: examination of the ‘state of control’ of the process using mean and range/standard deviation charts; a process capability study, comparing spread with specifications; process control using the charts.
