The recent reform of hospital financing increased the power of incentives producers respond to. In the meanwhile it made even more necessary to develop tools for productive efficiency analysis in the hospital sector. In this paper we measure technical efficiency of public hospital in Emilia-Romagna, between 1994 and 1995, with DEA (Data Envelopment Analysis). In particular, we investigate how sensitive results are with respect to the definition of output. Moreover we show how to test for significance in efficiency changes, as measured with DEA, by using both parametric and non parametric tests. Results are diffusely discussed and interpreted. The proposed approach, as far as it could lead to useful policy implications, seems definitely relevant.