The term common cause refers to a deviation from the normal in a project management system, but a deviation that is inherent and predictable. While this may seem contradictory, it actually makes tremendous sense. An example of a common cause, also referred to as a random cause, could be as follows: lightening strikes. Lightening strikes are inherently unpredictable. The location in which they strike is completely deviant. There is no consistency and no predictability in determining where they will strike. However, the fact that their strike locations cannot be predicted is predictable. And the fact that lightening will strike during a storm is predictable. Common causes, when displayed on a control chart, are indicated by random patterns of points within the otherwise set control limits. Common causes, when expected and prepared for, are rarely of any real negative consequence to a project team. However, if they are not taken into consideration during the project planning process, they can be more than a simple nuisance. They can cost time and money.
This term is defined in the 3rd and the 4th edition of the PMBOK.