This paper presents an overview of the field of recommender systems and describes the current generation of recommendation methods that are usually classified into the following three main categories: content-based, collaborative, and hybrid recommendation approaches. This paper also describes various limitations of current recommendation methods and discusses possible extensions that can improve recommendation capabilities and make recommender systems applicable to an even broader range of applications. These extensions include, among others, an improvement of understanding of users and items, incorporation of the contextual information into the recommendation process, support for multcriteria ratings, and a provision of more flexible and less intrusive types of recommendations.

Anomaly detection is an important problem that has been researched within diverse research areas and application domains. Many anomaly detection techniques have been specifically developed for certain application domains, while others are more generic. This survey tries to provide a structured and comprehensive overview of the research on anomaly detection. We have grouped existing techniques into different categories based on the underlying approach adopted by each technique. For each category we have identified key assumptions, which are used by the techniques to differentiate between normal and anomalous behavior. When applying a given technique to a particular domain, these assumptions can be used as guidelines to assess the effectiveness of the technique in that domain. For each category, we provide a basic anomaly detection technique, and then show how the different existing techniques in that category are variants of the basic technique. This template provides an easier and succinct understanding of the techniques belonging to each category. Further, for each category, we identify the advantages and disadvantages of the techniques in that category. We also provide a discussion on the computational complexity of the techniques since it is an important issue in real application domains. We hope that this survey will provide a better understanding of the di®erent directions in which research has been done on this topic, and how techniques developed in one area can be applied in domains for which they were not intended to begin with.

Collaborative filtering was initially proposed as a framework for filtering information based on the preferences of users, and has since been refined in many different ways. This thesis is a comprehensive study of rating-based, pure, non-sequential collaborative filtering. We analyze existing methods for the task of rating prediction from a machine learning perspective. We show that many existing methods proposed for this task are simple applications or modi cations of one or more standard machine learning methods for classifi cation, regression, clustering, dimensionality reduction, and density estimation. We introduce new prediction methods in all of these classes. We introduce a new experimental procedure for testing stronger forms of generalization than has been used previously. We implement a total of nine prediction methods, and conduct large scale prediction accuracy experiments. We show interesting new results on the relative performance of these methods.

We present a mixture model based approach for learning individualized behavior models for the Web users. We investigate the use of maximum entropy and Markov mixture models for generating probabilistic behavior models. We first build a global behavior model for the entire population and then personalize this global model for the existing users by assigning each user individual component weights for the mixture model. We then use these individual weights to group the users into behavior model clusters. We show that the clusters generated in this manner are interpretable and able to represent dominant behavior patterns. We conduct offline experiments on around two months worth of data from CiteSeer, an online digital library for computer science research papers currently storing more than 470,000 documents. We show that both maximum entropy and Markov based personal user behavior models are strong predictive models. We also show that maximum entropy based mixture model outperforms Markov mixture models in recognizing complex user behavior patterns. 1. Introduction and Related

Driven by successes in several application areas, maximum entropy modeling has recently gained considerable popularity. We generalize the standard maximum entropy formulation of classi cation problems to better handle the case where complex data distributions arise from a mixture of simpler underlying (latent) distributions. We develop a theoretical framework for characterizing data as a mixture of maximum entropy models. We formulate a maximum-likelihood interpretation of the mixture model learning, and derive a generalized EM algorithm to solve the corresponding optimization problem. We present empirical results for a number of data sets showing that modeling the data as a mixture of latent maximum entropy models gives signi cant improvement over the standard, single component, maximum entropy approach.

Web users display their preferences implicitly by navigating through a sequence of pages or by providing numeric ratings to some items. Web usage mining techniques are used to extract useful knowledge about user interests from such data. The discovered user models are then used for a variety of applications such as personalized recommendations. Web site content or semantic features of objects provide another source of knowledge for deciphering users ’ needs or interests. We propose a novel Web recommendation system in which collaborative features such as navigation or rating data as well as the content features accessed by the users are seamlessly integrated under the maximum entropy principle. Both the discovered user patterns and the semantic relationships among Web objects are represented as sets of constraints that are integrated to fit the model. In the case of content features, we use a new approach based on Latent Dirichlet Allocation (LDA) to discover the hidden semantic relationships among items and derive constraints used in the model. Experiments on real Web site usage data sets show that this approach can achieve better recommendation accuracy, when compared to systems using only usage information. The integration of semantic information also allows for better interpretation of the generated recommendations.

We describe a novel maximum entropy (maxent) approach for generating online recommendations as a user navigates through a collection of documents. We show how to handle high-dimensional sparse data and represent it as a collection of ordered sequences of document requests. Our representation and the maxent approach have several advantages: (1) we can naturally model long-term interactions and dependencies in the data sequences; (2) we can query the model quickly once it is learned, which makes the method applicable to highvolume Web servers; and (3) we obtain empirically high quality recommendations. Although maxent learning is computationally infeasible if implemented in the straightforward way, we explore data clustering and several algorithmic techniques to make learning practical even in high dimensions. We present several methods for combining the predictions of maxent models learned in different clusters. We conduct offline tests using over six months worth of data from ResearchIndex, a popular online repository of over 470,000 computer science documents. We show that our maxent algorithm is arguably one of the most accurate recommenders, as compared to such techniques as correlation, mixture of Markov models, mixture of multinomial models, individual similarity-based recommenders currently available on ResearchIndex, and even various combinations of current ResearchIndex recommenders.

Within the task of collaborative filtering two challenges for computing conditional probabilities exist. First, the amount of training data available is typically sparse with respect to the size of the domain. Thus, support for higher-order interactions is generally not present. Second, the variables that we are conditioning upon vary for each query. That is, users label different variables during each query. For this reason, there is no consistent input to output mapping. To address these problems we purpose a maximum entropy approach using a non-standard measure of entropy. This approach can be simplified to solving a set of linear equations that can be efficiently solved. 1

We present a novel approach to modeling sequences using mixtures of conditional maximum entropy distributions. Our method generalizes the mixture of first-order Markov models by including the "long-term" dependencies in model components. The "long-term" dependencies are represented by the frequently used in the natural language processing (NLP) domain probabilistic triggers or rules (such as "A occurred k positions back =) the current symbol is B with probability P "). The maximum entropy framework is then used to create a coherent probabilistic model from all triggers selected for modeling. In order to represent hidden or unobserved effects in the data we use probabilistic mixtures with maximum entropy models as components. We demonstrate how our mixture of conditional maximum entropy models can be learned from data using the EM algorithm that scales linearly in the dimensions of the data and the number of mixture components. We present empirical results on the simulated and real-world data sets and demonstrate that the proposed approach enables us to create better quality models than the mixtures of first-order Markov models and resist overfitting and curse of dimensionality that would inevitably present themselves for the higher order Markov models.

Abstract. We propose an approach for modeling the navigational behavior of Web users based on task-level patterns. The discovered “tasks” are characterized probabilistically as latent variables, and represent the underlying interests or intended navigational goal of users. The ability to measure the probabilities by which pages in user sessions are associated with various tasks, allow us to track task transitions and modality shifts within (or across) user sessions, and to generate task-level navigational patterns. We also propose a maximum entropy recommendation system which combines the page-level statistics about users ’ navigational activities together with our task-level usage patterns. Our experiments show that the task-level patterns provide better interpretability of Web users’ navigation, and improve the accuracy of recommendations. 1