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Incorporating Evidence into Trust Propagation Models Using Markov RandomFields
Hasari TosunDepartment of Computer Science
Montana State UniversityEPS 357, PO Box 173880Bozeman, MT 59717-3880
John W. SheppardDepartment of Computer Science
Montana State UniversityEPS 357, PO Box 173880Bozeman, MT 59717-3880
AbstractCurrent trust models for social networks com-monly rely on explicit voting mechanisms where individualsvote for each other as a form of trust statement. However,there is a wealth of information about individuals beyond trustvoting in emerging web based social networks. Incorporatingsources of evidence into trust models for social networks hasnot been studied to date. We explore a trust model for socialnetworks based on Markov Random Fields, which we callMRFTrust, that allows us to incorporate sources of evidence.To allow comparative evaluation, a state-of-the-art local trustalgorithmMoleTrustis also investigated. Experimental re-sults of the algorithms reveal that our trust algorithm thatincorporates evidence performs better in terms of coverage.It is competitive with the MoleTrust algorithm in predictionaccuracy and superior when focusing on controversial users.
Keywords-Trust Metrics, Reputation System, Social Network,Markov Random Fields
Recently, online Web services such as MySpace, Face-Book, Friendster, LiveJournal, Blogger, LinkedIn, Twitter,and Orkut have emerged as popular social networks. Thisnew generation of social networks is enormous, rich in infor-mation, and extremely dynamic. Moreover, in todays Web, avast amount of content is created by users. This content canrange from factual information to opinions about a person, aproduct, or a company. People constantly interact with otherpeople about whom they have no immediate information. Asa result, users of these services are constantly faced withquestions of how much they should trust the content createdor opinion provided by another person and how much theyshould trust the unknown person with whom he or she isabout to interact.
With this uncertainty in the mind, many e-commercecompanies such as eBay and Amazon enable users to rateother users or their reviews by providing a trust vote.Most online forums have some mechanism for users to rateothers opinions or responses. In some cases, the voting isimplicit. For example, reading an article can be consideredan implicit positive vote. Utilizing this vast amount of trustdata or aggregating trust scores for users have become a real
challenge for those companies. Trust and reputation is alsovery relevant to Peer-to-Peer (P2P) networks such as file-sharing networks. P2P networks are mainly used for sharingand distributing information. Thus, they are vulnerable to thespread of unauthentic files , , , . An alternativeutilization of the trust concept is used by the Google searchengine; a link from one web site to another is an expressionof trust .
As the Semantic Web gains acceptance, understanding thecredibility of metadata about authors is becoming important. While designing recommender systems, one researcherfound that there is a strong correlation between trust anduser similarity . Thus, trust became the essential variablein computing user similarity . Finally, trust concept isextensively applied to social networks. There is a wealthof information on trust and reputation scoring in social net-works , , , , . For example, Mui documentedthe theories and approaches about trust scores and reputationsystems using Bayesian networks for inference on socialnetworks .
There is no universal definition of trust and reputation.Barbalet characterized trust and its consequences in detail.He postulates that it is insufficient to define trust in termsof confident expectation regarding anothers behavior asmany researchers defined . Instead, the author char-acterizes trust in terms of acceptance of dependency (thetrust giver grants control or power to trustee; thus, thetrust giver accepts dependence on trustee) in the absense ofinformation about the others reliability in order to createan outcome otherwise unavailable. Golbeck and Hendleradopted a narrower definition of trust for social networkstrust in a person is a commitment to an action based onbelief that the future actions of that person will lead to agood outcome .
Although, researchers generally dont aggree on the def-inition of trust, two properties of trust are used for ag-gregation: transitivity and asymmetry. Transitivity meansif A trusts B who trusts C, then, A trusts C. Asymmetrymeans if A trusts B, it doesnt mean that B will also
3rd International Workshop on Security and Social Networking
978-1-61284-937-9/11/$26.00 2011 IEEE 263
trust A. The majority of trust propagation algorithms utilizethe transitivity property , , , , . It shouldbe noted this property may not always work with distrust. Moreover,  and  defined two types of trusts:referreral trust and direct functional trust. If A trusts Bwho trusts C, then, the trust between A-B and B-C isdirect functional trust. However, if B recomends C to Aas trustworthy, it is referral trust.
Modeling trust networks and propagating trust is a chal-lenging task: 1) trust networks are huge and sparse, and2) it is often difficult to model human belief and trust.Thus, researchers have often proposed simplistic approachesfor trust propagation. Ziegler and Lausen categorized trustmetrics on three dimensions : a network perspective,computation locus, and link evaluation. For the networkperspective, they categorized trust metrics as global andlocal. Global trust metrics consider all links and nodes inthe networks, where local trust metrics take into accountonly a partial network. Computational locus refers to theplace where trust relationships are computed. In determiningdistributed or local trust metrics, the computation load isdistributed to every node in the network, whereas in com-putationally centralized systems, all metric computations areperformed on a single machine. The tradeoff between thetwo approaches involves memory, performance, and security.Finally, link evaluation determines whether the trust metricsthemselves are scalar or group trust metrics. In group trustmetrics, trust scores are computed for a set of individualswhereas for scalar metrics only trust scores between twoindividuals are computed.
An increasing number of articles have been published onmodeling trust networks and evaluating trust metrics ,, , , , , , ,  using differentcomputational methods. For example, Wang and Vassilevadesigned a Bayesian Network-based trust model for P2P. The model represents different features of trust asleaf nodes of Naive Bayes networks. On the other hand, developed a model based on Fuzzy logic. Anotherpopular trust model is the Appleseed Trust metric based on aSpreading Activation Model , . Two different modelsbased on eigenvalue propagation were designed by  and.
Massa and Avesani studied challenges of computing trustmetrics in a social network where data are sparse .In such networks, neither a global reputation nor a simpletrust score is a viable option since a large percentage ofthe participants are considered to be controversial; theyare distrusted by some and trusted by others. Thus, theauthors proposed a state-of-the-art framework and algorithmcalled MoleTrust that uses local trust metrics. However, thisapproach does not incorporate other sources of evidence. Forexample, the epinion.com dataset contains articles writtenby users  where these articles are also rated by otherusers. Determining if such information is useful for a trust
algorithm is the challenge. We hypothesize that includingmore evidence into a trust model will improve the predictionpower of the model and its coverage.
The rest of the paper is organized as follows. Our trustprediction algorithm based on Markov Random Fields isdescribed in Section II. Then in Section III, we describethe dataset and present our results. The last section givesthe conclusions and future directions for our research.
II. METHODS AND PROCEDURESIn this paper, we describe our approach to developing
and using a trust network model based on Markov RandomFields (MRFs). A detailed introduction to MRFs is givenin . An MRF is a stochastic process that exhibits theMarkov property in terms of the interaction of neighboringnodes in the network. MRF models have a wide range ofapplication domains. The nodes in the MRF graph representrandom variables, and the edges represent the dependenciesbetween variables. In our approach, we use the same typeof model for propagating the trust scores in social networks.
The joint probability distribution over X and Y can berepresented by an MRF in the following way:
where Z is a normalization factor (also called the partitionfunction), (xi, xj) represents pairwise influence betweennode xi and xj in the network (often referred to as thepairwise compatibility matrix), and (xi, yi) is a localevidence function that forms a distribution over possiblestates, xi, given only its observations yi. When consideringthe application of MRFs to social network trust prediction,we note that the social network results in a trust networkwhenever users rate each other. Based on this observation,we developed a local algorithm for learning trust metrics byaugmenting an MRF representation of social networks withadditional sources of evidence. Our framework allows us toevaluate an active users trust for an unknown person in thenetwork.
There are two common methods for inference with MRFmodels : 1) M