Basic concepts

Definition: Method based on statistical techniques that are applied to control processes in order to:

1. monitor the process;

2. and detect unexpected causes of variation.

Causes of variation

• Natural or common cause: A probabilistically predictable source of variability, which is constantly active within the process. When only natural causes are present, the process is said to be in statistical control.
• Special cause: An inherently unpredictable source of variability. It means a change in one or more characteristics/components of the process, unexpected or previously neglected. When special causes appear, the process is not in control.