Special Cause Variation

Special cause variation refers to variation in an output or process that result from factors outside of the process. Special cause variation is often detected when output measures exceed the upper or lower control limits in a statistical process control chart. Special cause can also be detected when non-random patterns develop within the control limits.

An example of special cause of variation: Variation in the supply inputs for a process may change over time resulting in an process that goes out of statistical control.