Abstract
The main goal of the present paper is to study the general structure and theoretical properties of a particular type of a fuzzy measure that can be used to model multi criteria decision making problems in which there exist some sub criteria. After constructing the general form of the non-additive set function, we deal with the interaction coefficient, Möbius representation and dual measure related to proposed measure. Finally, we are concerned with the usage of this type of fuzzy measures in multi criteria decision making problems in which at least one of the criteria contains some sub-criteria.
Keywords
Introduction
Multi Criteria Decision Making (MCDM) is a process that ranks the alternatives or selects the best alternative under the conflicting criteria. Many researchers [9, 36] focused on the fuzzy studies related to the varied MCDM methods in order to obtain more realistic results since the concepts of decision making under fuzzy environments were proposed by Bellman and Zadeh [1]. Today, we meet numerous fuzzy-based MCDM studies [6–8]. A part of these studies deals with the fuzzy measure that is used as a tool to identify the weights of the criteria in many MCDM methods.
Related works
It is known that a fuzzy measure produces more factual results than additive ones. Thus, identification of a particular fuzzy measure is an important issue. In the literature, there are remarkable studies which consider fuzzy measure theory in MCDM problems [10, 41]. Although a fuzzy measure is such a useful tool for modelling the interaction of criteria in MCDM environment, the process of determining the measures of exponentially growing number of subsets of a finite set is complicated. Many authors studied fuzzy measure identification to obtain particular fuzzy measures [12, 39]. Besides, some authors used evolutionary or genetic algorithm to relieve this complexity [2, 38]. There are some general studies as well. For instance, Grabisch [13] introduced the concept of k-order additive fuzzy measures to control the calculation process of the measures of exponentially growing number of subsets.
Many popular MCDM problems such as supplier selection process, personnel selection process, website evaluation process, robot selection process etc. can include sub-criteria. To make clear the concept of sub-criterion, let us consider three main criteria: cost, acceleration, and quality as ranking the alternatives for a car selection problem. Should the cost criterion is divided into two criteria such as the cost of procurement and cost of maintenance, these two criteria are called the sub-criteria of the cost criterion and these structures are constructed to do more sensitive analysis in MCDM processes. Such problems can be evaluated with Analytic Hierarchy Process (AHP) [32] or Analytic Network Process (ANP) [33]. Besides, there are some works that study fuzzy measure theory in MCDM problems with sub-criteria. However, in these studies the authors have constructed known fuzzy measures to solve the problem. For instance, Yang et al. [42] considered λ-fuzzy measures in vendor selection problem that has some sub-criteria by obtaining the weights via AHP. Similarly, λ-fuzzy measures were considered in a supplier evaluation problem with sub-criteria by obtaining the weights via Fuzzy Analytic Hierarchy Process (FAHP) in [15]. Moreover, Takahagi [36] presented fuzzy measure identification methods by using the Ordered Weighted Average (OWA) operator and the λ-fuzzy measure for a hierarchy diagram. Wu and Beliakov [40] proposed the nonadditivity index to quantify the degree and kind of nonadditivity of a capacity, discussed some properties of this index, and presented some tools to help decision makers to determine the nonadditivity index of given subset. Krishnan et. al. [17] proposed an alternate version of λ0-measure identification method that synchronously compensates the shortcomings associated with each existing method. A supplier selection problem was used to demonstrate the feasibility of the method. Gong and Lei [14] integrated moving average with non-additive measures with σ - λ rules under fuzzy environment. There is not any direct study evaluating MCDM problems that contains sub-criteria in the realm of fuzzy measure theory so far. The main goal of this study is to construct the general form of a fuzzy measure that models MCDM problems allowing sub-criteria. The structure of the proposed set function is directly obtained from sub-criteria. In this context, this study fills a theoretical gap. Furthermore, many MCDM problems have multiple decision makers. Besides, many of these problems have large scale of decision makers. Moreover, in the large-scale group decision making, decision makers usually express their preferences using heterogeneous preference representation structures [43]. If we face such a problem the proposed measure still can be applicable. Further information about group decision making can be found in [23–25, 43].
Theoretical background
In this section we recall some definitions that are used in the present paper. Throughout the paper we use the notation AB instead of A ∪ B for any two sets A and B.
i) μ (∅) =0 and μ (X) =1
ii) μ (A) ≤ μ (B) whenever A ⊂ B ⊂ X (monotonicity)
where 2 X is the family of all subsets of X [11].
Note that the concept of fuzzy measure is a generalization of the concept of additive measures.
The following definition recalls the concept of Möbius representation [4] which is not only a useful tool to investigate the properties of the fuzzy measures but also characterizes them.
Grabisch [13] relaxed the concept of additivity by introducing the concept of k-order additivity. The motivation behind this new concept of additivity is to calculate the measures of at most k elements subsets to define a fuzzy measure. Now let us recall the definition of this concept:
Note here that k-additive fuzzy measures aresomehow between additive measures and general fuzzy measures, i.e., any fuzzy measure is k-order additive for some 1 ≤ k ≤ |X| and the worst scenario is the |X|-additivity of a fuzzy measure.
Another important concept to represent a fuzzy measure is interaction index [13]. Let X be a finite set and let μ be a fuzzy measure over X. The interaction index I (T) of a subset T of X is defined by
The remain of the paper is organized as follows: In Section 2, after introducing the fuzzy measure we investigate some properties of this measure. We calculate interaction coefficients, Möbius representation and dual measure related to the proposed measure. Section 3 is devoted for a hypothetical application. Finally, we conclude the paper in Section 4.
Main results
In this section we give the general structure and properties of a fuzzy measure that can be used in MCDM problems which contains sub-criteria. First of all, we introduce the theoretical part and then we give the connection of the theory with MCDM problems that allows sub-criteria.
is a fuzzy measure over
The following example shows that μ given in Theorem is not additive in general.
The following theorem gives the Möbius representation of a fuzzy measure μ given in Theorem with (1).
is the Möbius representation of the fuzzy measure μ given in Theorem where m j is the Möbius representation of μ j for each j.
Note that the Möbius representation of a set which is not a subset of one of the X
j
’s vanishes. This is not an unexpected result due to the Remark 1. Now we can say that a fuzzy measure defined with (1) is a k-order additive fuzzy measure for
It is possible to express the Shapley value of an element of S and interaction index of a subset of S with respect to μ in the terms of the Shapley values of an element of S and interaction indices of a subset of S, respectively, with respect to μ j for some j. For this purpose we recall the following result of Grabisch [13]:
and
where (ρ k ) is iterated sequence defined with
and α0 = 1.
if T ⊂ X j 0 and zero otherwise where I is the interaction representation of μ and I j 0 is the interaction representation of μ j 0 .
Let X be a finite set and let μ be a fuzzy measure on X. Then the set function μ∗ defined with
In this section, we give a hypothetical application in order to show the applicability of the proposed fuzzy measure on MCDM problems. Let us consider a personnel selection problem consisting of three main evaluation criteria and some sub-criteria: communication skills (x1) with sub-criteria (x11, x12); professional experience (x2); educational background (x3) with sub-criteria (x31, x32) (see Fig. 1). In the problem, the objective is to determine the best appropriate candidate based on these benefit criteria. For this purpose we deal with establishing a fuzzy measure on the set of criteria taking into account the hierarchy given in Fig. 1.

Hierarchy of considered criteria for a personnel selection problem.
For this MCDM problem, we can use a fuzzy measure that is given as (1). First of all we need a sequence
Fuzzy measures μ1, μ2, μ3 and μ t1
Clearly, the function μ is a fuzzy measure over S and it is not additive. However, it is 2-order additive, i.e., we can obtain the measure of a set that has at least three elements by adding up the measures of some smaller sets. Hence, it is enough to calculate the measures of pairs to determine the fuzzy measure. For example we see that μ ({ x11, x12, x2, x32 }) = μ ({ x11, x12 }) + μ ({ x2 }) + μ ({ x32 }). Note that defining the fuzzy measure μ we only calculate the measures of dbinom50 + dbinom51 + dbinom52 - 2 number of subsets.
This study introduces a new approach to use fuzzy measure theory in MCDM problems considering sub-criteria. Despite the previous works deal with fuzzy measure theory in MCDM problems containing sub-criteria, we give the general structure of a fuzzy measure which is not introduced before. First of all, we are concerned with a particular type of fuzzy measure that can be directly used in MCDM problems that contains sub-criteria. Then we calculate the interaction index, Möbius representation and dual fuzzy measure. We support our results with a numerical example.
Footnotes
Acknowledgement
We are thankful to the referees for their careful reading of the paper and several valuable suggestions which improved the presentation of the paper.
