A control chart is a tool that is useful for displaying and analyzing data that is produced over time. Control charts and run charts have been used in many industries for decades, and have been used in health care for several years, primarily for analysis of financial and administrative data. A control chart is essentially a run chart with the addition of upper and lower control limits that are generated for each chart from the data displayed on that chart. By displaying data in a graphical format, trends can be recognized that aren't immediately apparent from looking at a column of numbers. By additionally generating and displaying control limits on the chart, some rather elegant statistical analytical techniques can be used in a very user-friendly way.
All biological systems have inherent variation.
For example, a hypertensive patient's morning blood pressure varies considerably from day to day.