A source of variation which is non-random; a change in the source ("VITAL FEW" variables) will produce a significant change of some magnitude in the response (dependent variable), e.g., a correlation exists; the change may be due to an intermittent in-phase effect or a constant cause system which may or may not be highly predictable; an assignable cause is often signaled by an excessive number of data points outside a control limit and/or a non-random pattern within the control limits; an unnatural source of variation; most often economical to eliminate.

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."

A cause believed to be responsible for an identifiable change of precision or accuracy of a measurement process.