Slacks-Based Measure (SBM) models which was introduced by Tone (2001) put aside the assumption of proportionate changes in inputs and outputs, and deal with slacks directly. It has three variations, i.e., input-, output-, and non-oriented. The SBM models are designed to meet the following two conditions:
1. Units invariant: The measure should be invariant with respect to the units of data.
2. Monotone: The measure should be monotone decreasing in each slack in input and output.
Here, we introduce the SBM models in input-, output-, and non-oriented cases under the constant returns-to-scale assumption, but these models can be generalized to the variable returns-to-scale (VRS) environment. 

Input-Oriented SBM 

The input-oriented SBM efficiency model under the constant returns-to-scale assumption is defined by:

Output-Oriented SBM 

The output-oriented SBM efficiency model under the constant returns-to-scale assumption is defined by:

Non-Oriented SBM 

The non-oriented SBM efficiency model under the constant returns-to-scale assumption is defined by:

Non-oriented SBM can be transformed into a linear program using the Charnes–Cooper transformation as follows: