Scientific Programming

Volume 2015 (2015), Article ID 602690, 10 pages

http://dx.doi.org/10.1155/2015/602690

## A Community-Based Approach for Link Prediction in Signed Social Networks

^{1}Department of Computer System & Technology, Faculty of Computer Science & Information Technology, University of Malaya, 50603 Kuala Lumpur, Malaysia^{2}Advanced Community and Information System, RWTH Aachen University, Ahornstraße 55, 52056 Aachen, Germany

Received 28 February 2014; Accepted 8 October 2014

Academic Editor: Przemyslaw Kazienko

Copyright © 2015 Saeed Reza Shahriary et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

In signed social networks, relationships among nodes are of the types positive (friendship) and negative (hostility). One absorbing issue in signed social networks is predicting sign of edges among people who are members of these networks. Other than edge sign prediction, one can define importance of people or nodes in networks via ranking algorithms. There exist few ranking algorithms for signed graphs; also few studies have shown role of ranking in link prediction problem. Hence, we were motivated to investigate ranking algorithms availed for signed graphs and their effect on sign prediction problem. This paper makes the contribution of using community detection approach for ranking algorithms in signed graphs. Therefore, community detection which is another active area of research in social networks is also investigated in this paper. Community detection algorithms try to find groups of nodes in which they share common properties like similarity. We were able to devise three community-based ranking algorithms which are suitable for signed graphs, and also we evaluated these ranking algorithms via sign prediction problem. These ranking algorithms were tested on three large-scale datasets: Epinions, Slashdot, and Wikipedia. We indicated that, in some cases, these ranking algorithms outperform previous works because their prediction accuracies are better.

#### 1. Introduction

Recently, social network analysis has attracted great deal of attentions. In social networks, nodes and edges, respectively, indicate people and relationships among them [1]. Social networks are dynamic and evolve over time via registering new members, deleting profiles, and adding/removing some edges or connections among entities [2]. Hence, plenty of studies have investigated this field in order to model these structures. One of the most important problems in social networks is link prediction which can be stated as follows: with how much determinism one can predict forming (lacking) of edge between two people based on available structure of the graph? The importance of this subject is originated from the natural sparsity of social networks [2]. In other words, social networks encompass highly dynamic structure; therefore, available links are just a subset of possible relations among people and some new links will form in future. Link predication is also widely used in retrieving lost data and it probably helps to construct the graph [3].

One could model social systems by using signed relationships. Inherently in signed graphs, most of relations are positive or negative such as likes and dislikes or trusts and distrusts [4]. Negative edges play an important role in signed networks and these negative links impress greatly on importance of nodes in the system. Studying negative relationships in signed graphs can help in analyzing and better understanding social ecosystems. Link prediction in signed graph appears in the form of predicting sign of edge between two people. Therefore, one important question which comes to mind in signed networks is that how accurately sign of an edge can be predicted according to local and global behavioral patterns in the network. Not only sign prediction enables us to have better understanding of social relations but also it can be utilized in several applications such as recommender systems and online social networks, in which they offer new friends for users. In these networks, users have capability of expressing their views toward others via binary −1 and +1 values [5, 6].

In this paper, three community-based ranking algorithms for ranking of nodes have been proposed, and we have studied their impacts on the edge sign prediction problem. In order to study the impact of proposed ranking algorithms on signed prediction problem, we extracted the features of the predictor based on reputation and optimism introduced in [7]. Reputation of a node shows how much reputable a node is in the system and optimism denotes voting pattern of the node toward others. We assess our method by utilizing logistic regression classifier and running algorithms on real social network datasets. The structure of this paper is organized as follows.

In Section 2, related works are brought. In Section 3, we mainly introduce proposed algorithms; moreover, the problem of sign prediction is defined and rank-based features are introduced as features for the prediction task. We also separately go through community detection problem, community-based ranking algorithms, and the logistic regression classifier. In Section 4, datasets for experimental purposes are introduced and implementation results are also demonstrated. In Section 5, the discussion is made and finally in Section 6, conclusion and future directions are mentioned.

#### 2. Related Works

There are two major categories of methods used in link prediction: firstly, those approaches that utilize local information of the graph which focus on the local structure of nodes. Among local approaches, [8] has the best performance in link prediction between two specific nodes. Common neighbor index is also known as friend of friend algorithm (FOAF) is used by many online social networks for recommending friends such as Facebook. FOAF determines the similarity of two nodes that tend to communicate with each other on the basis of counting number of joint neighbors [9]. Other metrics for computing similarity are based on preferential attachments, where these measures are calculated based on multiplying or summing of nodes degree. Second category concentrates on global structure of the network and detecting overall features in order to find how strongly two nodes are similar. There are also diverse global approaches which use the whole adjacency matrix in order to predict hidden links, for instance, shortest path algorithm, PWR algorithm, and SimiRank algorithm [1, 10].

In sign prediction, the most notable and remarkable methods are divided into two categories: Belief Matrix Model [5] and machine learning approaches [11]. Belief Matrix Model was introduced by [5] and was proposed for predicting trust or distrust between two particular users in signed networks. It was the fundamental model in sign prediction of edges. Reference [11] employed the idea of signed triads and used logistic regression model and some local feature in order to predict sign of edges in social networks. The features that were introduced by [11] are categorized in two classes: first one is on the base of the positive/negative ingoing/outgoing degree of nodes which basically collect the local information of nodes. And the second group is based on the extracted principles from social psychology, in which we are able to determine the type of and relation by utilizing the information of third party like .

Ranking of nodes has tight relationship with sign prediction problem so we also investigate ranking in signed networks. Ranking of nodes is the problem of computing how much important or trustable a node is in networks [12]. The centrality measures like betweenness [13], closeness [14], and eigenvector centrality [15] were introduced to compute nodes’ importance degree in the network. Other algorithms like HITS [16] and PageRank [17] were added in 1990. All of these ranking algorithms are designed for positive graphs and there are merely several literatures for ranking of nodes in signed networks. The simplest ranking algorithm for signed graphs is prestige, where number of positive and negative incoming links determine ranking of each node [18]. Another ranking algorithm is PageTrust that was introduced by [19]. This method is extension of PageRank, and the main difference is that nodes with negative incoming links will be visited less in random walk process. Exponential ranking is another chief method of ranking for signed graphs [12]. In exponential ranking, the value of ranking vector globally is obtained from local trust values. Another ranking algorithm for signed networks that is greatly similar to HITS was proposed by [20]. This method utilizes the concept of Bias and Deserve which underestimates the vote of optimistic and pessimistic nodes. Reference [3] also proposed new ranking algorithms for signed networks, namely, Modified HITS and Modified PageRank.

Because we propose community-based ranking algorithms, we should go through community detection problem. Community detection algorithms help to prepare more dominant recommendation systems and web page clustering which have great effect on better searches [21]. Community detection algorithms attempt to cluster edges/nodes in order to have minimum number of edges between densely communities [22]. One of the most widely used methods for community detection in unsigned graphs was proposed by [23]. As for signed networks, [24] proposed a two-step spectral approach which was an extension to modularity. The main problem related to modularity is resolution limit in which very small communities might not be detected. In order to address this problem, [22] proposed new method for detecting communities on signed graphs by extending potts model. Reference [25] also introduced useful approach that works on the base of blocking method.

#### 3. Method

In this paper, authors intend to investigate the community-based problem of predicting sign of links in signed social networks. Hence, in this section as well as proposed algorithms and methods, the problems of sign prediction and community detection will be discussed in detail.

##### 3.1. Edge Sign Prediction

In order to define the problem formally, it can be assumed that we have a signed directed graph that represents set of vertices and shows set of edges where customers and users can vote positively and negatively toward each other. So the aforementioned notation represents users of site and indicates +1 and −1 relations among them. In all over the paper, the person who gives positive vote and receives it, is named trustor and trustee, respectively [2, 26]. The sign prediction problem can be defined as follows. Suppose that signs of some links in the network are hidden, and the goal is to reliably predict values of these edges by current information in the graph. The sign prediction problem tries to find signs of hidden edges with negligible error [1, 27]. In this work we propose state-of-the-art community-based ranking algorithms and we evaluate their effectiveness via sign prediction problem on three datasets: Epinoins, Slashdot, and Wikipedia.

To this end, [7] already introduced rank-based features named Optimism and Reputation to connect ranking problem with sign prediction. Rank-based reputation of node indicates patterns of voting toward this node. Meaningfully, rank-based reputation of node not only considers number of positive/negative incoming links toward node but also it takes into account ranks of nodes who vote toward node . In other words, when a person receives several positive incoming links, s/he might not be very reputable because one should consider rank of voters toward node . If the users who vote toward node are high rank, then node can be considered reputable, but if they are not high rank we cannot say that node is reputable although the number of positive incoming links toward node is relatively high. The following equation can better describe rank-based reputation [7]:where is the value of rank-based reputation of node , indicates sum of rank values of nodes who positively voted toward node , and, similarly, is sum of rank values of nodes who negatively voted toward node . In the same vein, one can define rank-based optimism of node as follows:where is the value of rank-based optimism of node , refers to sum of rank values of nodes whom node positively voted toward them, and similarly, is sum of rank values of nodes in which node negatively voted toward them. As formula (2) shows, node which generates several positive outgoing links might not be optimistic because this set might contain nodes in which they are low rank [3]. In order to compute these features, we need algorithms to rank nodes. As for ranking algorithms, we propose three community-based ranking algorithms in the next sections.

##### 3.2. Community-Based Ranking Algorithms to Compute RBR and RBO

In this section we propose three ranking algorithms in which all of them work based on community detection problem in signed graphs. In other words, firstly, we run a community detection algorithm on signed networks. The results will be disjoint communities of nodes. As all community detection algorithms work based on a density based approach in which they try to maximize density of intracluster edges and minimize between cluster edges, so intracluster nodes are more dense and close. From social perspective, intracluster nodes might know each other better (this is the notion behind our community-based ranking algorithm). Meaningfully, nodes in the same community are much more familiar than nodes that are in different communities. Via using this philosophy about intracluster nodes, we change previous ranking algorithm like Prestige, HITS, and PageRank [3] to have influence of intra- and extracluster nodes with parameters and (), respectively. Then we can use ranking-based features of [7] for the case of sign prediction. Because first phase of the algorithms is community detection, so we investigate community detection problem and a sample community detection algorithm in signed graphs in Section 3.2.1. In this paper a community detection algorithm based on social balance theory is utilized. In Section 3.3, ranking algorithms based on community detection phase are introduced.

###### 3.2.1. Community Detection

The algorithm used in this paper is based on structural balance theory [28]. In balance theory, there are four possible states when nodes are in signed relations in social networks [29]. One can differentiate these states by number of positive and negative edges in each triad [30]. On the base of strong social balance theory, when all of nodes have positive relation or two nodes share the same enemy, these states are called stable. Similarly, cases with all nodes have negative edges or with two positive edges are unstable states [31]. Regarding this definition, a network with more than three nodes is structurally balanced if all the possible triads are stable [32] (Figure 1).