In Express QC, phases can be used in a variable control chart to display how a process behaves in response to a change in a particular characteristic of the system. For example, outside factors can cause disruptions in the normal process, such as a construction project in a hospital that could cause additional in-patient falls.
Phase analysis can be helpful in identifying special causes of variation or trends. It is also useful when studying the impact of changes you make to your process, changes which you hope will be improvements.
Here is an example of a phase chart showing the changes that came about after new guidelines were established for shortening patient wait time. You can see how the horizontal lines representing the upper control limit, center line and lower control limit change with the new guidelines.