Definition

A process is said to be "in control" if its control chart has no points falling outside the region bounded by the Upper and Lower Control Limits and no unnatural patterns of variability show up on any part of the chart. Such a process is also called a 'stable' process. A stable or in-control process exhibits only common cause variation and its output is therefore predictable. It is desirable to run experiments and tests on a stable process, so that the conclusions are not muddied by unpredictable process variation.