The applications of the extent of separation and the extent of purification to multistage separation processes have been studied in detail. Neither index is suitable for stage-to-stage calculations in such processes. This is due to the fact that, at any stage, both indices depend only on the outlet stream compositions. It is not possible to reconstruct the inlet stream compositions knowing only the equilibrium compositions of the outlet streams, i.e., both indices represent an underspecification of the single stage within a multistage system.

A new index, E_m= abs X₁₁/X₂₂ X₁₁/X₂₂ is proposed. The application of this new index to a multistage distillation process gives a much more realistic comparison of the contribution of individual stages to the observed separation. Applications to other types of multistage separation processes as well as a single stage system have also been studied.