Control Chart - Sample Mean Chart & Sample Range Chart
As sample values are progressively plotted on the charts, they are examined for patterns which may be tested against a set of rules-of- thumb, as shown in the figure overleaf. This figure shows some examples of limits being exceeded. For example, if a single sample means breaches an action limit, the probability (1-in- 1000) that this would be obtained from a stable process is so low it is deemed to be more likely that the process has changed. The process should be stopped and investigated. If within any sequence of 10 plots you find one above the upper warning limit and one below the lower warning limit, this is statistically so unlikely that you should stop the process to investigate. Similarly, the occurrence of two consecutive sample means breaching a warning limit is so unlikely that, again, the process would be stopped and investigated.