The variations that occur in and around a nominal task time. (http://www.toc.co.uk/glossary.htm)
Statistical Fluctuation – (from The Critical Path E-Mail Newsletter) To understand TOC [Theory of Constraints], you must first comprehend that the activities of any company, be they production or development, contain two basic phenomena: 1) statistical fluctuation and 2) interdependent events (again linking to cause and effect). What does that mean? Basically, anything you do will have variable results, such as in production, how many parts can be fabricated in an hour? You don’t know for certain, but you do know within a range. Hence, statistical fluctuation. This might be affected by the availability of components from inventory, you may have too many sitting there and piling up, or worse – not enough, which causes delay. Hence, interdependent events.