Deciding on an suitable Action Level

The Action level is a percentage value used to determine when a process is out of control. Usually your entry will be based on experience. As you decrease the Action level more samples will fall outside the warning and action level lines plotted on the Control chart.

The theoretical value for the action level is calculated as:

action level = mean ± 3.09 x standard error of the mean

Where:

standard error of the mean = standard deviation/square root of the number of samples in a group

A value of 99.9 means that if a process is in control only one sample in a thousand would be expected to be outside the action level. This means that unless a large number of samples have been taken, any sample plotted outside the upper or lower action level strongly suggests the process is out of control. That is, assignable causes of variation are present.