A special cause refers to a system used in project management. A special cause, also called an assignable cause, is any factor or factors which may affect a system either in progress or outcome. There are always bound to be variances in a system, there are so many minute factors in any system. Environmental changes, wearing of tools, the accuracy of any human component, the frequency or lack of maintenance on necessary machinery, they all effect a system. Typically these crate variations in the function of a system or its outcome in a stable, repeatable, and predictable pattern over time. These are called common causes and may be taken into account by project management. A Special cause however, is not predictable or stable. That is the problem; as such it may only be assigned by project management personal as a defect in the system. Such special causes may be indicated or detected by control charts. Points of variance beyond control limits, or non-random patterns shown within the control limits are indicators of a special cause. Because of the unpredictable nature of a special cause, it cannot be taken into account by project management, requiring a complete inspection and rethink of the system in order to find the defect and fix it.
This term is defined in the 3rd and the 4th edition of the PMBOK.