Statistical Process Control is a methodology for improving business processes. One of the most widely known tools used with the methodology are statistical control charts such as X-bar and Individual-Moving Range. These charts use statistical methods to assess whether a business process operates within a stable range of variance. The charts are also often used to identify special or common cause variation in a process.
A statistical control chart typically sets upper and lower control limits (often at the third standard deviations above and below the mean for the sample). When data points exceed these control limits, the process is considered to be out of statistical control (and experiencing special cause variation.) Often, analysts will also employ other tests to determine whether a process is out of control by looking for various non-random patterns in the process data.