(1) Outside influences on the process's performance that change the nature and add variation to a process. One should look for assignable causes for points outside the control limits or points inside the limits with unnatural patterns.
(2) (also special cause) A source of variation that is not part of the inherent variation for which it is often possible to assign the reason for the variation. OR A name for the source of variation in a process that is not due to chance and therefore can be identified and eliminated. Also called "special cause."
(3) An assignable cause is a source of variation that is intermittent, not predictable. It is sometimes called "special cause" variation. On a control chart, an assignable cause is signaled by points beyond the control limits or nonrandom patterns within the control limits.