This paper introduces a slacks-based measure (SBM) of super-efficiency in Data Envelopment Analysis (DEA). The SBM, proposed by the author in a previous paper, differs from traditional radial measures by directly addressing input/output slacks. The paper discusses the rationality of the SBM by comparing it with radial measures, particularly the Andersen and Petersen model. The SBM is shown to be units invariant and monotone decreasing in input/output slacks. The paper also specializes the SBM to input and output orientation, demonstrating its effectiveness through numerical examples. The SBM is compared with the Andersen and Petersen model, highlighting the irrationality of the latter's treatment of input slacks. The paper concludes by discussing future research directions, including the exploration of other distance norms and the dual side of associated linear programs.This paper introduces a slacks-based measure (SBM) of super-efficiency in Data Envelopment Analysis (DEA). The SBM, proposed by the author in a previous paper, differs from traditional radial measures by directly addressing input/output slacks. The paper discusses the rationality of the SBM by comparing it with radial measures, particularly the Andersen and Petersen model. The SBM is shown to be units invariant and monotone decreasing in input/output slacks. The paper also specializes the SBM to input and output orientation, demonstrating its effectiveness through numerical examples. The SBM is compared with the Andersen and Petersen model, highlighting the irrationality of the latter's treatment of input slacks. The paper concludes by discussing future research directions, including the exploration of other distance norms and the dual side of associated linear programs.