One of the objectives of Quality Assurance is to reduce the variability in process execution.

Common Causes of Variation

All processes contain some inherent variation, or common causes of variation. The amount of variation in a process is quantified with summary statistics (the mean and standard deviation).

A process is defined as stable when its mean and standard deviation remain constant over time. Processes containing only common causes of variation are considered stable. As a stable process is predictable, future process values can be predicted within the control limits with a certain amount of belief. A stable process is said to be in a state of statistical control.

One researcher1 provides the following thoughts on common causes of variation:

"Process inputs and conditions that regularly contribute to the variability of process outputs.”

“Common causes contribute to output variability because they themselves vary.”

Special Causes of Variation

Special causes of variation are not present in a process. They occur because of special or unique circumstances. In the IT example of abnormal terminations in a computer operation, special causes might include operator strikes, citywide power outages, or earthquakes.

If special causes of variation exist, the process may be unpredictable, and therefore unstable. A state of statistical control is established when all special causes of variation have been eliminated (the operator strike ends, citywide power returns or business returns to normal operations after an earthquake).

Brian Joiner summarized special causes of variation as follows:

"Process inputs and conditions that sporadically contribute to the variability of process outputs.”

“Special causes contribute to output variability because they themselves vary.”