Measuring efficiency at U.S. banks: accounting for heterogeneity is important

Estimates of bank cost efficiency can be biased if bank heterogeneity is ignored. The author compares X-inefficiency measures derived from a model that constrains the cost frontier to be the same for all banks in the nation and a model that allows the cost functions and error terms to differ across Federal Reserve Districts. The author finds that the data reject the single cost function model; X-inefficiency measures based on the single cost function model are, on average, higher than those based on the separate cost functions model; the distributions of the one-sided error terms on which X-inefficiency measures are based are wider for the single cost function model than for the separate cost functions models; and the ranking of Districts by the level of X-inefficiency differs in the two models. The differences in efficiency across Districts reflect more than just differences in bank size, geographic size, or population of the Districts. These results suggest that it is important when studying X-inefficiency to account for differences across the markets in which banks are operating and, more generally, that since X-inefficiency is, by construction, a residual, it will be particularly sensitive to omissions in the basic model.