“…mix of labour or capital resources; five DEA models considered both single and disaggregated measures of inputs (see additional file 1). Inputs were measured in physical units in either total working hours or working hours by skill level [23,36,38,40,41,47,56], or in absolute number of individuals by skill level [21,22,25,29,32,33,51,53]. Only four studies divided their input mix into two categories (i.e.…”