A department often begins and ends a cost period with partially processed units of product, so the completed units in the department are not an accurate measure of the department's total production.

Indeed, the very notion of productivity is a by-product of the clock: once one can relate performance to uniform time units, work is never the same.

Companies that must determine well in advance of the selling season how many units of a new product to manufacture often underproduce products that sell well and have overstocks of others.