(1) A source of variation that is acting on or common to all outcomes of a process. It is constantly present but its influence may vary over time.
(2) Natural or random variation that is inherent in a process over time, affecting every outcome of the process. If a process is in control it has common cause variation only.
(3) A source of variation that is inherent in a system and is predictable. A control chart identifies a system with only common causes of variation. Common causes of variation affect all individual values of a system, and can be eliminated only by a systemic change.