FDH

The free disposal hull (FDH) model originally was designed as an alternative to data envelopment analysis (DEA) models, where only the strong (free) disposability of inputs and outputs is assumed. The main contribution was to relax the convexity assumption of DEA models. So we have an observed unit as a reference instead of a convex combination of them which is not observed.

Leleu, H., 2006, "A linear programming framework for free disposal hull technologies and cost functions: Primal and dual models," European Journal of Operational Research, 168, 340-344.