Are overhead costs strictly proportional to activity? : Evidence from hospital departments

Abstract Using cross-sectional data from hospitals in Washington State, we test whether overhead costs are proportional to overhead activities. This assumption is at the heart of nearly all cost accounting systems, which implicitly assume that marginal cost is equal to average cost. Empirically, the proportionality hypothesis can be rejected for most of the overhead accounts. On average across the accounts, the average cost per unit of activity overstates marginal costs by about 40% and in some departments by over 100%. Thus, the average cost per activity should be used with a great deal of caution in decisions.