TY - JOUR AU - Rostamy-Malkhalifeh,, Mohsen AB - Abstract Inverse (DEA) is an approach to estimate the expected input/output variation levels when the efficiency score reminds unchanged. Essentially, finding most efficient decision-making units (DMUs) or ranking units is an important problem in DEA. A new ranking system for ordering extreme efficient units based on inverse DEA is introduced in this article. In the adopted method, here the amount of required increment of inputs by increasing the outputs of the unit under evaluation is obtained through the proposed models. By obtaining these variations, this proposed methodology enables the researcher to rank the efficient DMUs in an appropriate manner. Through the analytical theorem, it is proved that suggested models here are feasible. These newly introduced models are validated through a data set of commercial banks and a numerical example. 1. Introduction Data envelopment analysis (DEA) is a method for calculating the performance evaluation of the decision-making units (DMUs) of specific inputs and outputs. Charnes et al. (1978) introduced DEA for the first time to measure the efficiency of the DMUs. DEA models are based on mathematical programming for estimating the efficiency units. The relative efficiency of each one of the units will be obtained by solving a linear programming (LP) problem. The inverse DEA is introduced by Wei et al. (2000) to estimate the output variation subject to an increase in inputs and preservation of the efficiency score. Wei et al. (2000) applied the multiple-objective programming (MOP) to estimate output levels. Jahanshahloo et al. (2004a,c) and Yan et al. (2002) use the decision makers’ preferences to convert multiple-objective LP (MOLP) into a single-objective LP. In addition to the above models, there exist some other proposal like Yan et al. (2002) and Li & Cui (2008) where the input for resource allocation is estimated. Hadi-Vencheh & Foroughi (2006) introduced a model for increasing some input levels and decreasing others. Lertworasirikul et al. (2011) presented a model regarding the concurrent increases of some outputs and decreases of other outputs. Jahanshahloo et al. (2014) studied the inverse DEA using the non-radial enhanced Russell model. Jahanshahloo et al. (2015) introduced a periodic weak Pareto solution for MOLP to solving inverse DEA problems and proposed inverse DEA subject to inter-temporal dependence assumption. Hadi-vencheh et al. (2015) extended the inverse DEA models for interval data in evaluating the efficiency score. Ghobadi & Jahangiri (2015) developed fuzzy inverse DEA models. Gattoufi et al. (2014) applied inverse DEA for a case of merger and acquisition in banking in order to obtain the required level of inputs and outputs of the merged bank to reach a given efficiency score. Afterwards, Amin et al. (2017) proposed a new method to foretasted whether a merger in a market is generating a major or a minor consolidation, through inverse DEA model. Some of the latest research on inverse DEA are shown in Table 1. Table 1. Some of the latest inverse DEA research Reference Short description Amin et al. (2019) Using goal programming and inverse DEA for target setting merger Emrouznejad et al. (2018) Allocating CO2 emission quota under several assumptions Kalantary & Saen (2018) Inverse network DEA model in dynamic context for evaluation Sustainability of supply chains Amin & Al-Muharrami (2018) Using inverse DEA in merging units with negative data by inverse DEA Ghobadi (2018) Using enhanced Russell model for fuzzy data Amin et al. (2017) Using inverse DEA in merge units Ghiyasi (2017) Using inverse DEA when data prices is available for assessing cost efficiency Reference Short description Amin et al. (2019) Using goal programming and inverse DEA for target setting merger Emrouznejad et al. (2018) Allocating CO2 emission quota under several assumptions Kalantary & Saen (2018) Inverse network DEA model in dynamic context for evaluation Sustainability of supply chains Amin & Al-Muharrami (2018) Using inverse DEA in merging units with negative data by inverse DEA Ghobadi (2018) Using enhanced Russell model for fuzzy data Amin et al. (2017) Using inverse DEA in merge units Ghiyasi (2017) Using inverse DEA when data prices is available for assessing cost efficiency Open in new tab Table 1. Some of the latest inverse DEA research Reference Short description Amin et al. (2019) Using goal programming and inverse DEA for target setting merger Emrouznejad et al. (2018) Allocating CO2 emission quota under several assumptions Kalantary & Saen (2018) Inverse network DEA model in dynamic context for evaluation Sustainability of supply chains Amin & Al-Muharrami (2018) Using inverse DEA in merging units with negative data by inverse DEA Ghobadi (2018) Using enhanced Russell model for fuzzy data Amin et al. (2017) Using inverse DEA in merge units Ghiyasi (2017) Using inverse DEA when data prices is available for assessing cost efficiency Reference Short description Amin et al. (2019) Using goal programming and inverse DEA for target setting merger Emrouznejad et al. (2018) Allocating CO2 emission quota under several assumptions Kalantary & Saen (2018) Inverse network DEA model in dynamic context for evaluation Sustainability of supply chains Amin & Al-Muharrami (2018) Using inverse DEA in merging units with negative data by inverse DEA Ghobadi (2018) Using enhanced Russell model for fuzzy data Amin et al. (2017) Using inverse DEA in merge units Ghiyasi (2017) Using inverse DEA when data prices is available for assessing cost efficiency Open in new tab One of the most controversial issues in DEA is concerned with ranking DMUs. There exist many articles in the literature regarding ranking efficient units. Some methods regarding ranking DMUs are categorized as cross efficiency, super efficiency, techniques based on finding optimal weight, multi-criteria decision making (MCDM) technique, reference-based approaches, etc. The first category involves evaluation of the cross efficiency matrix where a unit is a self- and peer-evaluated. In this method, the efficiency of each DMU is evaluated |$n$| times compared with the other DMUs in the set of data. Results are provided on the cross-efficiency matrix, which its main diagonal elements are the self-efficiency for each DMU. Sexton et al. (1986) introduced the cross-efficiency method and is provided by Rodder & Reucher (2011), Jahanshahloo et al. (2011), Ramon et al. (2011), Contreras (2012), Wu et al. (2016a) and Wu et al. (2016b). An et al. (2018) incorporated the DEA and analytic hierarchy process (AHP) methods to fully rank the DMUs that consider all possible cross efficiencies of a DMU according to all the other DMUs. Furthermore, Liu (2018) consider cross efficiency intervals and their variances for ranking DMUS. Supper efficiency models are based on the idea that one unit is excluded from the production possibility set (PPS) and is assessed through the remaining other units. This idea has been a subject of study by Andersen & Petersen (1993), Mehrabian et al. (1999), Tone (2002), Jahanshahloo et al. (2004b), Amirteimoori et al. (2005), Jahanshahloo et al. (2006), Li et al. (2007), Rezai Balf et al. (2012) and Chen et al. (2013). The ranking units are based on the optimal weights obtained from the multiplier model of DEA. Washio & Yamada (2013), Wang et al. (2009), Alirezaee & Afsharian (2007), Liu & Peng (2008) and Lotfi et al. (2011) have developed this method. Oukil (2018) introduced a new approach by combining cross efficiency, weighting schemes and ordered weighted averaging (OWA) operator to provide a full ranking of DMUs. He used the property of multiple weighting schemes generated over the cross-evaluation process for developing a methodology that gives not only robust ranking patterns but also more realistic sets of weights for the DMUs. The MCDM technique has been studied by Cook & Kress (1991), Strassert & Prato (2002), Chen (2007) and Jablonsky (2012). Reference-based approaches assume the importance of efficient DMUs as references for other DMUs to assign a criterion for discriminating among efficient DMUs. Charnes et al. (1984), Jahanshahloo et al. (2007) and Chen & Deng (2011) have extended this approach. Recently, Rezaeiani & Foroughi (2018) proposed a model to measure the reference frontier share for ranking efficient units. Some of the latest research on ranking are shown in Table 2. Table 2. Some of the latest ranking research Reference Short description Liu et al. (2019) Investigate the cross-efficiency evaluation in DEA based on prospect theory de Blas et al. (2018) Applying a novel ranking method to measure dominance derived from social network analysis by detection method of self-evaluations Mufazzal & Muzakkir (2018) A new method of ranking based on proximity indexed value Hatami-Marbini et al. (2018) Introducing the SRDM measures regardless of feasibility or infeasibility of the model Liu & Wang (2018) Constructing the interval efficiency evaluation by the normalized efficiency and achieve the best normalized efficiency and the worst normalized efficiency for ranking. Lin & Chen (2018) Introducing a new modified input-oriented variable returns to scale supper-efficiency (VRS SE) model and its corresponding algorithm. Wang & Sun (2018) Dividing the problem of DMUs’ nonhomogeneity into external nonhomogeneity and internal homogeneity Salehian et al. (2018) A novel hybrid algorithm based on fuzzy analytical hierarchy process and DEA Reference Short description Liu et al. (2019) Investigate the cross-efficiency evaluation in DEA based on prospect theory de Blas et al. (2018) Applying a novel ranking method to measure dominance derived from social network analysis by detection method of self-evaluations Mufazzal & Muzakkir (2018) A new method of ranking based on proximity indexed value Hatami-Marbini et al. (2018) Introducing the SRDM measures regardless of feasibility or infeasibility of the model Liu & Wang (2018) Constructing the interval efficiency evaluation by the normalized efficiency and achieve the best normalized efficiency and the worst normalized efficiency for ranking. Lin & Chen (2018) Introducing a new modified input-oriented variable returns to scale supper-efficiency (VRS SE) model and its corresponding algorithm. Wang & Sun (2018) Dividing the problem of DMUs’ nonhomogeneity into external nonhomogeneity and internal homogeneity Salehian et al. (2018) A novel hybrid algorithm based on fuzzy analytical hierarchy process and DEA Open in new tab Table 2. Some of the latest ranking research Reference Short description Liu et al. (2019) Investigate the cross-efficiency evaluation in DEA based on prospect theory de Blas et al. (2018) Applying a novel ranking method to measure dominance derived from social network analysis by detection method of self-evaluations Mufazzal & Muzakkir (2018) A new method of ranking based on proximity indexed value Hatami-Marbini et al. (2018) Introducing the SRDM measures regardless of feasibility or infeasibility of the model Liu & Wang (2018) Constructing the interval efficiency evaluation by the normalized efficiency and achieve the best normalized efficiency and the worst normalized efficiency for ranking. Lin & Chen (2018) Introducing a new modified input-oriented variable returns to scale supper-efficiency (VRS SE) model and its corresponding algorithm. Wang & Sun (2018) Dividing the problem of DMUs’ nonhomogeneity into external nonhomogeneity and internal homogeneity Salehian et al. (2018) A novel hybrid algorithm based on fuzzy analytical hierarchy process and DEA Reference Short description Liu et al. (2019) Investigate the cross-efficiency evaluation in DEA based on prospect theory de Blas et al. (2018) Applying a novel ranking method to measure dominance derived from social network analysis by detection method of self-evaluations Mufazzal & Muzakkir (2018) A new method of ranking based on proximity indexed value Hatami-Marbini et al. (2018) Introducing the SRDM measures regardless of feasibility or infeasibility of the model Liu & Wang (2018) Constructing the interval efficiency evaluation by the normalized efficiency and achieve the best normalized efficiency and the worst normalized efficiency for ranking. Lin & Chen (2018) Introducing a new modified input-oriented variable returns to scale supper-efficiency (VRS SE) model and its corresponding algorithm. Wang & Sun (2018) Dividing the problem of DMUs’ nonhomogeneity into external nonhomogeneity and internal homogeneity Salehian et al. (2018) A novel hybrid algorithm based on fuzzy analytical hierarchy process and DEA Open in new tab Based on our literature review on ranking methods there is a lack of study for DMU’s extension and new features. To compensate this lack, we have utilized the inverse DEA approach for the aim. In the classical inverse DEA models, the DMU with existing inputs and outputs is applied for building PPS. In this article, a model is based on removing the current unit and replacing it with a revised DMU consisting of renewed inputs and outputs is introduced. The new unit is applied for building the possibility set. In this method, through MOLP model the amount of required increment of the inputs are obtained when the outputs of the unit under evaluation increase and the unit remains efficient. By obtaining this variation, this proposed method (PM) enables the efficient DMUs to be ranked in an appropriate manner. Finding a unique optimal solution for MOLP models is a challenging task; thus, a single-objective LP model is presented here to face this challenge. The inverse DEA method is the futuristic approach for ranking DMUs due to its ability to estimate the inputs changes based on any increments of the outputs so is named the growth potential. This article is structured as follows: some prerequisite from inverse DEA are given in Section 2. The correlation between inverse DEA and changing PPS is discussed in Section 3. An inverse DEA approach for ranking efficient extreme DMUs is proposed in Section 4. Numerical examples to illustrate the applicability of this PMs are presented in Section 5 and the conclusions are presented in Section 6. 2. Prerequisite A MOP is defined as follows: $$\begin{equation} \begin{array}{l} {~~~~~ Min f(x)} \\{s.t. ~~~g(x) \leqslant 0,} \\ \end{array} \end{equation}$$ (2.1) where |$ f:R^m \, \rightarrow R^q \,\,\ $| and |$ \,\ g:R^m \, \rightarrow R^p $| are the two vector functions, (i.e. |$ f(x)=(\,f_1(x),f_2(x),\dots ,f_q(x)) $|⁠, and |$ g(x)=(g_1(x),g_2(x),\dots ,g_p(x)) $|⁠). The set |$ X=\{x\in R^m: g_k(x)\leqslant 0,~~ k=1,2,\dots ,p\} $| is named the set of feasible solutions of MOP introduced by model (2.1). Provided that |$ f(.) $| and |$ g(.) $| are linear functions, model (2.1) is renamed MOLP. Usually, there is no solution |$ x \in X $| to minimize all objective functions; hence, the Pareto solutions should be considered for this purpose instead of optimal solutions. The definitions of the Pareto solution are as follows (Ehrgott, 2005): Definition 2.1 Assume |$ \bar{x} \in X $| is a feasible solution of MOP (2.1), |$ \bar{x} $| is named a Pareto solution provided that there does not exist |$ x \in X $| in a sense that |$ f_i(x) \leqslant f_i(\bar{x}) \,\ $| for all |$i=1,2,\dots ,q,$||$ f_i(x) \ < f_i(\bar{x})$| for some |$i=1,2,\dots ,q$|⁠. Definition 2.2 Assume |$ \bar{x} \in X $| is a feasible solution of MOP (2.1), |$ \bar{x} $| is named a weak Pareto solution provided that there does not exist |$ x \in X $| such that |$ f_i(x) \ < f_i(\bar{x}) \,\ $| for all |$i=1,2,\dots ,q$|⁠. Assume a set of DMUs, {|$DMU_j$|⁠: |$j = 1, 2,\dots , n $|}, which produce multiple-output |$y_{rj} (r =1, 2,..., s)$|⁠, through multiple-input |$x_{ij} (i =1, 2,..., m)$|⁠. The inputs and outputs of |$DMU_j$| are represented by vectors: |$X_{j} =(x_{1j}, x_{2j},..., x_{mj})$| and |$Y_{j} = ( y_{1j}, y_{2j},..., y_{sj})$|⁠, respectively. The PPS |$ T_{c} $| is defined as $$\begin{equation*} T_{c}=\left \{ (X,Y) \,: \, X \geqslant \sum _{j=1}^{n}X_{j} \lambda _{j} ~~,~~ Y \leqslant \sum _{j=1}^{n}Y_{j} \lambda _{j}~,~~ \lambda_j \geqslant 0,~~ j=1, \dots,n \right \}. \end{equation*}$$ For evaluating the efficiency of |$DMU_o$|⁠, |$ o \in{\{1,2,..., n\}}$| the following DEA model should be applied: $$\begin{equation} \begin{array}{l} {~~~~~ Min \theta} \\{s.t. ~~~\sum\limits _{j=1}^{n}x_{ij} \lambda _{j} \leqslant \theta x_{io}, \,\,\,\,\,\,\,\,\ i=1,2,\dots,m,} \\[12pt] {~~~~~~~\sum\limits _{j=1}^{n}y_{rj} \lambda _{j} \geqslant y_{ro}, \,\,\,\,\,\,\,\,\,\ r=1,2,\dots,s, } \\[12pt] ~~~~~~~~\lambda _{j} \geqslant 0,\ ~~~~~~~~~~~~~~j=1,2,\dots,n. \end{array} \end{equation}$$ (2.2) This model is named after Charnes, Cooper and Rhodes are abbreviated as CCR (Charnes et al., 1978). The efficiency under a constant returns to scale (CRS) assumption is measured by this model. Assume that |$\theta ^{*}$| is the optimal solution (efficiency index) of CCR model, it is obvious that |$\theta ^{*} \leqslant 1$|⁠. If |$\theta ^{*} = 1$|⁠, then |$DMU_o$| is efficient. Now consider the following question: ‘How much would the inputs increase when the outputs increase at unchanged efficiency score of |$\theta $|?’ To answer this question, assume outputs of |$DMU_o$| increase from |$ Y_o $| to |$ Y_{o} +K_o $|⁠, where the vectors |$ Y_{o}\geqslant 0 $|⁠, |$Y_{o} \ne 0 $| and |$ K_o \in R_+^s$|⁠; the inputs’ vectors |$X_{o}+\varDelta X_o$| must be estimated such that the efficiency index of |$DMU_o$| remains |$\theta ^{*}$| unchanged. The vectors |$ Y_{o} +K_o $| and |$X_{o}+\varDelta X_o$| are represented by |$ \beta _o $| and |$ \alpha _o $|⁠, respectively. The following MOLP model is applied for measuring |$ \alpha _o $| (Wei et al., 2000): $$\begin{equation} \begin{array}{l} { Min(\alpha_{1o},\alpha_{2o},\dots,\alpha_{mo})} \\{s.t. ~~\sum\limits _{j=1}^{n}x_{ij} \lambda _{j} \leqslant \theta^* \alpha_{io}, \,\,\,\,\,\,\,\,\,\,\,\ i=1,2,\dots,m,} \\[12pt] {~~~~~~\sum\limits _{j=1}^{n}y_{rj} \lambda _{j} \geqslant \beta_{ro}, ~~~~~~~~~~~~ r=1,2,\dots,s,} \\[12pt] {~~~~~~ \alpha_{io} \geqslant x_{io}, ~~~~~~~~~~~~~~~~~~~~ i=1,2,\dots,m, }\\ ~~~~~~\lambda \in \varLambda. \end{array} \end{equation}$$ (2.3) where $$\begin{equation*} \varLambda=\left\{\lambda|\lambda=(\lambda_1,\ldots,\lambda_n,\lambda_{n+1}),~\delta_1\left(\sum_{j=1}^{n+1}\lambda_j+\delta_2(-1)^{\delta_3}\nu\right)=\delta_1, ~\nu\geqslant0,~\lambda_j\geqslant 0,j=1,\ldots,n+1\right\}\!. \end{equation*}$$ It can be easily observed that |$\bullet $| if |$ \delta _1=0 $|⁠, model (2.3) is the CCR model. |$\bullet $| if |$ \delta _1=1 $| and |$ \delta _2=0 $|⁠, model (2.3) is the Banker, Charnes, and Cooper (BCC) model. |$\bullet $| if |$ \delta _1= \delta _2=1 $| and |$ \delta _3=0 $|⁠, model (2.3) is the CCR-BCC model. |$\bullet $| if |$ \delta _1= \delta _2= \delta _3=1 $|⁠, model (2.3) is the BCC-CCR model. After changing the inputs and outputs into |$\alpha ^{*}_o=X_o +\varDelta X_{o}^* $| and |$\beta _o= Y_{o}+K_o $|⁠, respectively, where |$ \alpha _{o}^{*} $| is a Pareto solution of model (2.3), the |$DMU_{o}$| is named |$DMU_{n+1}$|⁠, the efficiency of which is measured by the following model: $$\begin{equation} \begin{array}{l} {Min\ \theta} \\[4pt] {s.t. ~~~~ \sum\limits_{j=1}^{n}x_{ij} \lambda _{j}+ \lambda_{n+1} \alpha^{*}_{io} \leqslant \theta \alpha^{*}_{io},\,\,\,\,\,\,\,\ ~~ i=1,2,\dots,m, } \\[10pt] {~~~~~~~~\sum\limits _{j=1}^{n}y_{rj} \lambda _{j} +\lambda_{n+1} \beta_{ro} \geqslant \beta_{ro}, \,\,\,\,\,\,\,\,\,\,\,\,\,\ r=1,2,\dots,s, } \\ ~~~~~~~~\lambda \in \varLambda. \end{array} \end{equation}$$ (2.4) The optimal value of problem (2.4) is |$\theta ^{*}_1$|⁠, and the optimal value of (2.2) is |$\theta ^{*}$| and in case |$\theta ^{*}_1 =\theta ^{*}$|⁠, the efficiency will remain unchanged. Hadi-Vencheh et al. (2008), proved that problem (2.3) preserves the efficiency index through the following theorem. Definition 2.3 Assume |$(\alpha _{o}^{*}, \lambda ^{*}) $| is a feasible solution of problem (2.3); it is deducted that |$(\alpha _{o}^{*}, \lambda ^{*}) $| is a Pareto solution (strongly efficient solution) of problem (2.3), if and only if there is no feasible solution |$(\alpha _{o}, \lambda ) $| of (2.3) where |$ \alpha _{io}^{*} \geqslant \alpha _{io} $| for all |$i=1,2,\dots ,m$| and |$ \alpha _{io}^{*} \;> \alpha _{io} $| for at least one i. (Hadi-Vencheh et al., 2008) Definition 2.4 Assume |$(\alpha _{o}^{*}, \lambda ^{*}) $| is a feasible solution of problem (2.3), it is deducted that |$(\alpha _{o}^{*}, \lambda ^{*}) $| is a weakly efficient solution of (2.3), if and only if there is no feasible solution |$(\alpha _{o}, \lambda ) $| of (2.3) where |$ \alpha _{io}^{*} \;> \alpha _{io} $| for all |$i=1,2,\dots ,m$| (Hadi-Vencheh et al., 2008). Theorem 2.1 Assume the optimal value of problem (2.2) is |$\theta ^{*}$| and |$(\alpha ^{*}_{o}, \lambda ^{*}) $| be a strongly efficient solution of problem (2.3), then the optimal value of problem (2.4) is |$\theta ^{*}$|⁠, when the inputs of DMUs are increased up to |$X_{o}+\varDelta X_o$| (Hadi-Vencheh et al., 2008). 3. Inverse DEA and PPS change In this section, we propose a new model with consideration the changing PPS and apply it to ranking DMUs. The traditional inverse DEA models yield a new DMU. In these models, the PPS is constructed by previous DMUs, that is, it is built by |$(DMU_{1}, DMU_{2},$||$\dots ,DMU_{o},\dots DMU_{n})$|⁠, while the |$DMU_o$| is changed to a fresh unit named |$DMU^{^{\prime}}_{o}$|⁠. Hadi-vencheh et al. (2015) presented a new approach by eliminating |$DMU_o$| to obtain the inputs gain and they constructed the PPS by |$(DMU_{1}, \dots ,DMU_{o-1},$||$DMU_{o+1}\dots ,DMU_{n})$|⁠. This method is applied in interval data. In general, PPS can be constructed by substituting the ‘perturbed DMU’ with a modified unit containing the refreshed inputs and outputs. In this study, the |$DMU_o(X_{o},\,Y_{o})$| is substituted with |$DMU^{^{\prime}}_{o}(\alpha _o,\,\beta _o)$|⁠, therefore, the PPS is constructed by |$(DMU_{1}, \dots ,DMU_{o-1}, DMU^{^{\prime}}_{o},DMU_{o+1}\dots ,DMU_{n} )$|⁠. For this purpose, the following model is applied to obtain |$ \alpha _o $|⁠: $$\begin{equation} \begin{array}{l} {Min(\alpha_{1o},\alpha_{2o},\dots,\alpha_{mo})} \\[4pt] {s.t.~~\sum\limits _{j\ne o}^{n}x_{ij} \lambda _{j} \leqslant \theta \alpha_{io}, ~~~~~ i=1,2,\dots,m,} \\[10pt] { ~~~~~~\sum\limits _{j\ne o}^{n}y_{rj} \lambda _{j} \geqslant \beta_{ro}, ~~~~~~ r=1,2,\dots,s, } \\[10pt] {~~~~~~\alpha_{io} \geqslant x_{io}, ~~~~~ i=1,2,\dots,m, } \\[4pt] ~~~~~~~ \lambda \in \varLambda. \end{array} \end{equation}$$ (3.1) Note: among the existing DMUs, the unchanged one (⁠|$ DMU_o $|⁠) cannot create a new PPS. Model (3.1) under CRS is change to the following model: $$\begin{equation} \begin{array}{l} {Min(\alpha_{1o},\alpha_{2o},\dots,\alpha_{mo})} \\[4pt] {s.t.~~\sum\limits _{j\ne o}^{n}x_{ij} \lambda _{j} \leqslant \theta \alpha_{io}, ~~~~~ i=1,2,\dots,m,} \\[10pt] { ~~~~~~\sum\limits _{j\ne o}^{n}y_{rj} \lambda _{j} \geqslant \beta_{ro}, ~~~~~~ r=1,2,\dots,s, } \\[10pt] {~~~~~~\alpha_{io} \geqslant x_{io}, ~~~~~ i=1,2,\dots,m, } \\[4pt] ~~~~~~~ \lambda_j \geqslant 0 ~~~~~~~~~~~~~~~ j=1,2,\dots,n. \end{array} \end{equation}$$ (3.2) Theorem 3.1 Model (3.2) is always feasible. Proof. See Appendix A. The efficiency score of |$DMU^{^{\prime}}_{o}$| is evaluated by the following model: $$\begin{equation} \begin{array}{l} {Min \theta} \\[4pt] {s.t.~~~\sum\limits_{j\ne o}^{n} x_{ij} \lambda _{j} +\lambda_{o} \alpha^{*}_{io} \leqslant \theta \alpha^{*}_{io}, \,\,\,\,\,\,\,\,\,\ i=1,2,\dots,m, } \\[10pt] {~~~~~~~\sum\limits_{j\ne o}^{n} y_{rj} \lambda _{j} +\lambda_{o} \beta_{ro} \geqslant \beta_{ro}, \,\,\,\,\,\,\,\,\,\,\ r=1,2,\dots,s, } \\[10pt] ~~~~~~~\lambda _{j} \geqslant 0,\, \, \, \, j=1,2,\dots,n. \end{array} \end{equation}$$ (3.3) Example 3.2 Consider five DMUs |$ \{A,B,C,D,E\} $| under variable returns to scale technology according to Table 3. Assume the output of unit B increases by |$k=0.5$| amount and the increment of input |$ (\varDelta x=0.625) $| is obtained through traditional inverse DEA (model (2.3)). The revised DMU with |$ (x+\varDelta x, y+k) $| component is named |$ B^{\prime} $| as shown in Fig. 1. As can be seen, PPS does not change. Yet, if the proposed model (model (3.1)) is used for calculating input increment, then |$ (\varDelta x=1.188) $|⁠. Hence, unit B is changing into |$ B^\prime $|⁠, as shown in Fig. 2(a); therefore, PPS changes too. New PPS has been built with the participation of |$ B^{\prime} $| as shown in Fig. 2(b). In other words, unit B is removed and instead of that, unit |$ B^{\prime} $| is used to build PPS. Table 3. Inputs and outputs DMUs A B C D E |$x $| 1.5 2.5 5 4 7 |$ y $| 2 4 6 3 6 DMUs A B C D E |$x $| 1.5 2.5 5 4 7 |$ y $| 2 4 6 3 6 Open in new tab Table 3. Inputs and outputs DMUs A B C D E |$x $| 1.5 2.5 5 4 7 |$ y $| 2 4 6 3 6 DMUs A B C D E |$x $| 1.5 2.5 5 4 7 |$ y $| 2 4 6 3 6 Open in new tab Fig. 1. Open in new tabDownload slide Finding new DMU by traditional inverse DEA. Fig. 1. Open in new tabDownload slide Finding new DMU by traditional inverse DEA. Fig. 2. Open in new tabDownload slide Changing PPS. Fig. 2. Open in new tabDownload slide Changing PPS. If technology production is CRS assumption, then two cases may happen for PPS that is shown in Figs 3 and 4. First, suppose the only efficient unit is B as shown in Fig. 3. Assume the output of unit B increases by |$k=0.5$| amount, if the proposed model (model (3.2)) is used for calculating input increment, then |$ (\varDelta x=0.875) $|⁠. Hence, unit B is changing into |$ B^\prime $| as shown in Fig. 3; therefore, PPS changes too. New PPS has been built with the participation of |$ B^{\prime} $||$( OAB^{\prime}) $|⁠. Second, if B and C are efficient units, then they are on a line as a frontier. By inverse DEA model the unit B changes into |$ B^{\prime} $| that is on the same efficient frontier again and PPS is not changing (Fig. 4). Fig. 3. Open in new tabDownload slide Changing CCR-PPS. Fig. 3. Open in new tabDownload slide Changing CCR-PPS. Fig. 4. Open in new tabDownload slide CCR-PPS. Fig. 4. Open in new tabDownload slide CCR-PPS. To solve model (3.2) there exist different technique like interactive methods, goal programming and the sum weighted method. In this article, the sum weighted method is applied for this purpose. The Pareto solutions of model (3.2) preserves efficiency, that is, |$DMU^{^{\prime}}_o$| with |$(\alpha ^{*}_{o},\,\beta _{o}) $| is of the efficiency score one |$(\theta ^{*}=1)$|⁠. This model is applied only for efficient units; accordingly, the following theorem is evolved. Theorem 3.3 Assume the optimal value of problem (2.2) for |$ DMU_o $| is |$\theta ^{*}=1$|⁠, and |$(\alpha ^{*}_{o}, \lambda ^{*}) $| is a strongly efficient solution of problem (3.2), then the efficiency score of |$ DMU^{^{\prime}}_o (\alpha ^{*}_{o},\,\beta _o) $| is one. Proof. Let |$(\alpha _{o}^{*}, \lambda ^{*}) $| be a Pareto solution (strongly efficient solution) of problem (3.2) and |$(\theta ^{*}_{1}, \lambda ^{\prime}) $| be an optimal solution of (3.3), then |$\theta ^{*}_{1}=1$| must be proved. By assuming $$\begin{equation*}\bar \lambda_{j}= \begin{cases} \lambda^{*}_{j}, & \mbox{if} ~~j \ne o,\\ 0, & \mbox{if } ~~ j=o, \end{cases} \end{equation*}$$ it becomes clear that |$(\bar \lambda ,\theta ^{*}_{1} )$| is a feasible solution of problem (3.3). Since |$(\alpha _{o}^{*}, \lambda ^{*}) $| is the solution of (3.2), the following is yielded through input restrictions of problem (3.3): $$\begin{equation*}\theta^{*}_{1} \alpha^{*}_{io} \geqslant \sum_{j\ne o}^{n} x_{ij} \lambda^{\prime} _{j} + \lambda^{\prime}_{o} \alpha_{io} \geqslant \sum_{j\ne o}^{n} x_{ij} \lambda^{\prime} _{j}+ \lambda^{\prime} _{o} \,\sum _{j\ne o}^{n}x_{ij} \lambda^{*} _{j} = \sum _{j\ne o}^{n}x_{j} ( \lambda^{*} _{j} \lambda^{\prime}_{o} + \lambda^{\prime}_{j} ),\end{equation*}$$ the same procedure hold true for output restriction: $$\begin{equation*}y_{ro} \leqslant y_{ro}+k_{r} \leqslant \sum _{j\ne o}^{n}y_{rj} ( \lambda^{*} _{j} \lambda^{\prime}_{o} + \lambda^{\prime}_{j}), \end{equation*}$$ the above mentioned variables are briefed as |$ \tilde \lambda _{j} = ( \lambda ^{*} _{j} \lambda ^{\prime}_{o} + \lambda ^{\prime}_{j}) $| for |$ j \ne o $|⁠, and for |$j=o$| set |$ \tilde \lambda _{o}=0 $|⁠. Assume that |$ \theta ^{*}_{1} \;< 1$|⁠. In this case, the following two scenarios can happen: |$\bullet $| (a) if |$ \alpha ^{*}_{o} = X_{o} $|⁠, with respect to |$ \theta ^{*}_{1} \;< 1$|⁠, and |$(\tilde \lambda ,\theta ^{*}_{1} )$| as a feasible solution of problem (2.2), then the solution |$ \theta ^{*}_{1} \;< 1$| is impossible because |$ \theta ^{*}=1 $| is the optimal solution of (2.2). |$\bullet $| (b) If |$ \alpha ^{*}_{o} \gneq X_{o} $| then |$\sum \limits _{j\ne o}^{n} x_{ij} \tilde \lambda _{j} \leqslant \theta ^{*}_{1} \alpha ^{*}_{io} \leqslant \alpha ^{*}_{io} $|⁠. There exists at least one i, such that |$ \alpha ^{*}_{io} \;> x_{io} $|⁠; the following definition: $$\begin{equation*}h=min \Big\{min \Big\{ \frac{ \theta^{*}_{1} \alpha^{*}_{io} - \sum_{j\ne o}^{n} x_{ij} \tilde\lambda _{j}} { \theta^{*}_{1}} \Big\}, min\{\alpha^{*}_{io} -x_{io}: \alpha^{*}_{io} -x_{io} \;> 0 \}\Big\}, \end{equation*}$$ let $$\begin{equation*}\bar \alpha_{io} = \begin{cases} \alpha^{*}_{io}, & \mbox{if} ~~~ \alpha^{*}_{io} =x_{io}, \\ \alpha^{*}_{io} -h, & \mbox{if } ~~~ \alpha^{*}_{io} \;> x_{io}, \\ \end{cases}\end{equation*}$$ thus $$\begin{equation*}h \leqslant \frac{ \theta^{*}_{1} \alpha^{*}_{io} - \sum_{j\ne o}^{n} x_{ij} \tilde\lambda _{j}} {\theta^{*}_{1}} \, \Rightarrow \, \sum_{j\ne o}^{n} x_{ij} \tilde\lambda _{j} \leqslant \theta^{*}_{1} (\alpha^{*}_{io} - h) \leqslant \theta^{*}_{1} \bar \alpha_{io}, \end{equation*}$$ and $$\begin{equation*}\alpha^{*}_{io} -x_{io} \geqslant h \, \, \Rightarrow \alpha^{*}_{io} -h \geqslant x_{io} \, \Rightarrow \, \alpha^{*}_{io} -h \;> 0.\end{equation*}$$ Consequently, |$ (\alpha ^{*}_{o},\tilde \lambda ) $| is a feasible solution of problem (2.3) subject to |$ \bar \alpha _{io} \leqslant \alpha _{io}^{*} $| for all |$i=1,2,\dots ,m$|⁠, and |$ \bar \alpha _{io} \;< \alpha _{io}^{*} $| for some |$i=1,2,\dots ,n$|⁠; while there exists a contradiction to the assumption that |$(\alpha _{o}^{*}, \lambda ^{*}) $| is a strongly efficient solution of problem (3.2). Thus, |$ \theta ^*_1 =1$|⁠. 4. Ranking system with inverse DEA In this section, we use the inverse DEA models under CRS technology due to the following: (i) in one of the models, all outputs of a unit are increased by a similar amount, so needs a model that is stable to dimension changes. (ii) The model under VRS results in infeasible for some units while CCR model is feasible. Consider that |$ DMU_o $| with the inputs of |$ x_{io} $| (⁠|$i=1,2,\dots ,m$|⁠) and outputs of |$ y_{ro} $| (⁠|$r=1,2,\dots ,s$|⁠) is an efficient unit. Assume that |$ y_{ro} $| changes into |$ y_{ro}+k_r $|⁠, where $$\begin{equation*} \begin{cases} k_r \leqslant l_{r}, & \mbox{if }~~y_{rj} \geqslant 1 ~~ \mbox{for all j,}\\ k_r \leqslant \min\limits_j(y_{rj}), & \mbox{if } ~~ 0