AbstractData envelopment analysis (a mathematical programming technique) has often been applied to measuring the efficiency with which outputs are produced. The technique derives efficient combinations of outputs for given inputs: constant returns to size may be assumed or one may choose to examine whether decreasing or increasing returns hold true. An analysis of the cost of prescribing drugs for 106 general practices in the Lincolnshire Health Authority for the year 1993/1994 reveals the statistical problems that are encountered in applying this technique.