The Mean & Range Chart is used to monitor the performance of a process where the feature under investigation is a variable. The monitoring is done by taking samples of the process's outputs at predetermined intervals and measuring each item in the sample. Both the size of the sample and the frequency with which it is taken must be decided before plotting begins and clearly stated in the Chart headings.
The required measurement from each item is then recorded on the chart and calculations carried out to find the mean and range values for the sample.
The mean (average) of each sample is calculated by the formula:
Mean = Sum x / n
where n is the sample size.
The value is then recorded in the appropriate box and plotted on the Mean graph.
The range of each sample is then calculated by subtracting the smallest value from the largest. This figure is then recorded in the appropriate box and plotted on the Range graph.