Blohm, I.; Ott, F.; Bretschneider, U.; Huber, M.; Rieger, M.; Glatz, F.; Koch, M.; Leimeister, J. M. & Krcmar, H.: Extending Open Innovation Platforms into the real world -
ing Large Displays in Public Spaces. 10. European Academy of Management Conference (EURAM) 2010. Rome, Italy: 2010
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Blohm, I.; Ott, F.; Bretschneider, U.; Huber, M.; Rieger, M.; Glatz, F.; Koch, M.; Leimeister, J. M. & Krcmar, H.: Extending Open Innovation Platforms into the real world - Using Large Displays in Public Spaces. 10. European Academy of Management Conference (EURAM) 2010. Rome, Italy: 2010
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Gemmell, J.; Schimoler, T. R.; Christiansen, L. & Mobasher, B.: Improving Folkrank With Item-Based Collaborative Filtering. In: Jannach, D.; Geyer, W.; Freyne, J.; Anand, S. S.; Dugan, C.; Mobasher, B. & Kobsa, A. (Hrsg.): ACM RecSys'09 Workshop on Recommender Systems and the Social Web. 2009 (CEUR-WS.org 532), S. 17-24
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Collaborative tagging applications allow users to annotate online resources. The result is a complex tapestry of interrelated users, resources and tags often called a folksonomy. Folksonomies present an attractive target for data mining applications such as tag recommenders. A challenge of tag recommendation remains the adaptation of traditional recommendation techniques originally designed to work with two dimensional data. To date the most successful recommenders have been graph based approaches which explicitly connects all three components of the folksonomy. In this paper we speculate that graph based tag recommendation can be improved by coupling it with item-based collaborative filtering. We motive this hypothesis with a discussion of informational channels in folksonomies and provide a theoretical explanation of the additive potential for item-based collaborative filtering. We then provided experimental results on hybrid tag recommenders built from graph models and other techniques based on popularity, user-based collaborative filtering and item-based collaborative filtering. We demonstrate that a hybrid recommender built from a graph based model and item-based collaborative filtering outperforms its constituent recommenders. furthermore the inability of the other recommenders to improve upon the graph-based approach suggests that they offer information already included in the graph based model. These results confirm our conjecture. We provide extensive evaluation of the hybrids using data collected from three real world collaborative tagging applications.

Koren, Y.: Collaborative filtering with temporal dynamics. Proceedings of the 15th ACM SIGKDD international conference on Knowledge discovery and data mining. New York, NY, USA: ACM, 2009, S. 447-456
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Customer preferences for products are drifting over time. Product perception and popularity are constantly changing as new selection emerges. Similarly, customer inclinations are evolving, leading them to ever redefine their taste. Thus, modeling temporal dynamics should be a key when designing recommender systems or general customer preference models. However, this raises unique challenges. Within the eco-system intersecting multiple products and customers, many different characteristics are shifting simultaneously, while many of them influence each other and often those shifts are delicate and associated with a few data instances. This distinguishes the problem from concept drift explorations, where mostly a single concept is tracked. Classical time-window or instance-decay approaches cannot work, as they lose too much signal when discarding data instances. A more sensitive approach is required, which can make better distinctions between transient effects and long term patterns. The paradigm we offer is creating a model tracking the time changing behavior throughout the life span of the data. This allows us to exploit the relevant components of all data instances, while discarding only what is modeled as being irrelevant. Accordingly, we revamp two leading collaborative filtering recommendation approaches. Evaluation is made on a large movie rating dataset by Netflix. Results are encouraging and better than those previously reported on this dataset.

Liang, H.; Xu, Y.; Li, Y. & Nayak, R.: Tag Based Collaborative Filtering for Recommender Systems. In: Wen, P.; Li, Y.; Polkowski, L.; Yao, Y.; Tsumoto, S. & Wang, G. (Hrsg.): Rough Sets and Knowledge Technology . Berlin, Heidelberg: Springer Berlin Heidelberg, 2009 (5589), S. 666-673
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Collaborative tagging can help users organize, share and retrieve information in an easy and quick way. For the collaborative tagging information implies user's important personal preference information, it can be used to recommend personalized items to users. This paper proposes a novel tag-based collaborative filtering approach for recommending personalized items to users of online communities that are equipped with tagging facilities. Based on the distinctive three dimensional relationships among users, tags and items, a new similarity measure method is proposed to generate the neighborhood of users with similar tagging behavior instead of similar implicit ratings. The promising experiment result shows that by using the tagging information the proposed approach outperforms the standard user and item based collaborative filtering approaches.

Parra, D. & Brusilovsky, P.: Evaluation of Collaborative Filtering Algorithms for Recommending Articles on CiteULike. Proceedings of the Workshop on Web 3.0: Merging Semantic Web and Social Web. 2009 (CEUR Workshop Proceedings 467)
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Motivated by the potential use of collaborative tagging systems to develop new recommender systems, we have implemented and compared three variants of user-based collaborative filtering algorithms to provide recommendations of articles on CiteULike. On our first approach, Classic Collaborative filtering (CCF), we use Pearson correlation to calculate similarity between users and a classic adjusted ratings formula to rank the recommendations. Our second approach, Neighbor-weighted Collaborative Filtering (NwCF), incorporates the amount of raters in the ranking formula of the recommendations. A modified version of the Okapi BM25 IR model over users ’ tags is implemented on our third approach to form the user neighborhood. Our results suggest that incorporating the number of raters into the algorithms leads to an improvement of precision, and they also support that tags can be considered as an alternative to Pearson correlation to calculate the similarity between users and their neighbors in a collaborative tagging system.

Parra, D. & Brusilovsky, P.: Evaluation of Collaborative Filtering Algorithms for Recommending Articles on CiteULike. Proceedings of the Workshop on Web 3.0: Merging Semantic Web and Social Web. 2009 (CEUR Workshop Proceedings 467)
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Motivated by the potential use of collaborative tagging systems to develop new recommender systems, we have implemented and compared three variants of user-based collaborative filtering algorithms to provide recommendations of articles on CiteULike. On our first approach, Classic Collaborative filtering (CCF), we use Pearson correlation to calculate similarity between users and a classic adjusted ratings formula to rank the recommendations. Our second approach, Neighbor-weighted Collaborative Filtering (NwCF), incorporates the amount of raters in the ranking formula of the recommendations. A modified version of the Okapi BM25 IR model over users ’ tags is implemented on our third approach to form the user neighborhood. Our results suggest that incorporating the number of raters into the algorithms leads to an improvement of precision, and they also support that tags can be considered as an alternative to Pearson correlation to calculate the similarity between users and their neighbors in a collaborative tagging system.

Takács, G.; Pilászy, I.; Németh, B. & Tikk, D.: Scalable Collaborative Filtering Approaches for Large Recommender Systems. In: Journal of Machine Learning Research 10 (2009), S. 623-656
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The collaborative filtering (CF) using known user ratings of items has proved to be effective for predicting user preferences in item selection. This thriving subfield of machine learning became popular in the late 1990s with the spread of online services that use recommender systems, such as Amazon, Yahoo! Music, and Netflix. CF approaches are usually designed to work on very large data sets. Therefore the scalability of the methods is crucial. In this work, we propose various scalable solutions that are validated against the Netflix Prize data set, currently the largest publicly available collection. First, we propose various matrix factorization (MF) based techniques. Second, a neighbor correction method for MF is outlined, which alloys the global perspective of MF and the localized property of neighbor based approaches efficiently. In the experimentation section, we first report on some implementation issues, and we suggest on how parameter optimization can be performed efficiently for MFs. We then show that the proposed scalable approaches compare favorably with existing ones in terms of prediction accuracy and/or required training time. Finally, we report on some experiments performed on MovieLens and Jester data sets.

Jaeschke, R.; Marinho, L.; Hotho, A.; Schmidt-Thieme, L. & Stumme, G.: Tag Recommendations in Folksonomies. In: Hinneburg, A. (Hrsg.): Workshop Proceedings of Lernen - Wissensentdeckung - Adaptivität (LWA 2007). Martin-Luther-Universität Halle-Wittenberg, 2007, S. 13-20
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Jaeschke, R.; Marinho, L.; Hotho, A.; Schmidt-Thieme, L. & Stumme, G.: Tag Recommendations in Folksonomies. In: Hinneburg, A. (Hrsg.): Workshop Proceedings of Lernen - Wissensentdeckung - Adaptivität (LWA 2007). Martin-Luther-Universität Halle-Wittenberg, 2007, S. 13-20
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Schafer, J. B.; Frankowski, D.; Herlocker, J. & Sen, S.: Collaborative Filtering Recommender Systems. In: Brusilovsky, P.; Kobsa, A. & Nejdl, W. (Hrsg.): The Adaptive Web: Methods and Strategies of Web Personalization. Berlin, Heidelberg: Springer, 2007 (Lecture Notes in Computer Science 4321), S. 291-324
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One of the potent personalization technologies powering the adaptive web is collaborative filtering. Collaborative filtering (CF) is the process of filtering or evaluating items through the opinions of other people. CF technology brings together the opinions of large interconnected communities on the web, supporting filtering of substantial quantities of data. In this chapter we introduce the core concepts of collaborative filtering, its primary uses for users of the adaptive web, the theory and practice of CF algorithms, and design decisions regarding rating systems and acquisition of ratings. We also discuss how to evaluate CF systems, and the evolution of rich interaction interfaces. We close the chapter with discussions of the challenges of privacy particular to a CF recommendation service and important open research questions in the field.

Adomavicius, G. & Tuzhilin, A.: Toward the next generation of recommender systems: a survey of the state-of-the-art and possible extensions. In: Knowledge and Data Engineering, IEEE Transactions on 17 (2005), Nr. 6, S. 734-749

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 multicriteria ratings, and a provision of more flexible and less intrusive types of recommendations.

Herlocker, J.; Konstan, J.; Terveen, L. & Riedl, J.: Evaluating collaborative filtering recommender systems. In: ACM Transactions on Information Systems 22 (2004), Nr. 1, S. 5-53

Herlocker, J. L.; Konstan, J. A.; Terveen, L. G. & Riedl, J. T.: Evaluating collaborative filtering recommender systems. In: ACM Trans. Inf. Syst. 22 (2004), S. 5-53
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Recommender systems have been evaluated in many, often incomparable, ways. In this article, we review the key decisions in evaluating collaborative filtering recommender systems: the user tasks being evaluated, the types of analysis and datasets being used, the ways in which prediction quality is measured, the evaluation of prediction attributes other than quality, and the user-based evaluation of the system as a whole. In addition to reviewing the evaluation strategies used by prior researchers, we present empirical results from the analysis of various accuracy metrics on one content domain where all the tested metrics collapsed roughly into three equivalence classes. Metrics within each equivalency class were strongly correlated, while metrics from different equivalency classes were uncorrelated.

Whitby, A.; Jøsang, A. & Indulska, J.: Filtering Out Unfair Ratings in Bayesian Reputation Systems. 2004

Melville, P.; Mooney, R. J. & Nagarajan, R.: Content-boosted Collaborative Filtering for Improved Recommendations. Eighteenth National Conference on Artificial Intelligence. Menlo Park, CA, USA: American Association for Artificial Intelligence, 2002, S. 187-192
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Most recommender systems use Collaborative Filtering or Content-based methods to predict new items of interest for a user. While both methods have their own advantages, individually they fail to provide good recommendations in many situations. Incorporating components from both methods, a hybrid recommender system can overcome these shortcomings. In this paper, we present an elegant and effective framework for combining content and collaboration. Our approach uses a content-based predictor tc enhance existing user data, and then provides personalized suggestions through collaborative filtering. We present experimental results that show how this approach, <i>Content-Boosted Collaborative Filtering</i>, performs better than a pure content-based predictor, pure collaborative filter, and a naive hybrid approach.

Sarwar, B.; Karypis, G.; Konstan, J. & Riedl, J.: Item-based collaborative filtering recommendation algorithms. Proceedings of the 10th international conference on World Wide Web. New York, NY, USA: ACM, 2001, S. 285-295
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Sarwar, B.; Karypis, G.; Konstan, J. & Riedl, J.: Item-based collaborative filtering recommendation algorithms. WWW '01: Proceedings of the 10th International Conference on World Wide Web. New York, NY, USA: ACM, 2001, S. 285-295
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Recommender systems apply knowledge discovery techniques to the problem of making personalized recommendations for information, products or services during a live interaction. These systems, especially the k-nearest neighbor collaborative filtering based ones, are achieving widespread success on the Web. The tremendous growth in the amount of available information and the number of visitors to Web sites in recent years poses some key challenges for recommender systems. These are: producing high quality recommendations, performing many recommendations per second for millions of users and items and achieving high coverage in the face of data sparsity. In traditional collaborative filtering systems the amount of work increases with the number of participants in the system. New recommender system technologies are needed that can quickly produce high quality recommendations, even for very large-scale problems. To address these issues we have explored item-based collaborative filtering techniques. Item-based techniques first analyze the user-item matrix to identify relationships between different items, and then use these relationships to indirectly compute recommendations for users. In this paper we analyze different item-based recommendation generation algorithms. We look into different techniques for computing item-item similarities (e.g., item-item correlation vs. cosine similarities between item vectors) and different techniques for obtaining recommendations from them (e.g., weighted sum vs. regression model). Finally, we experimentally evaluate our results and compare them to the basic k-nearest neighbor approach. Our experiments suggest that item-based algorithms provide dramatically better performance than user-based algorithms, while at the same time providing better quality than the best available user-based algorithms.

Herlocker, J. L.; Konstan, J. A. & Riedl, J.: Explaining collaborative filtering recommendations. CSCW '00: Proceedings of the 2000 ACM Conference on Computer Supported Cooperative Work. New York, NY, USA: ACM, 2000, S. 241-250
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Automated collaborative filtering (ACF) systems predict a person's affinity for items or information by connecting that person's recorded interests with the recorded interests of a community of people and sharing ratings between like-minded persons. However, current recommender systems are black boxes, providing no transparency into the working of the recommendation. Explanations provide that transparency, exposing the reasoning and data behind a recommendation. In this paper, we address explanation interfaces for ACF systems - how they should be implemented and why they should be implemented. To explore how, we present a model for explanations based on the user's conceptual model of the recommendation process. We then present experimental results demonstrating what components of an explanation are the most compelling. To address why, we present experimental evidence that shows that providing explanations can improve the acceptance of ACF systems. We also describe some initial explorations into measuring how explanations can improve the filtering performance of users.

Kautz, H.; Selman, B. & Shah, M.: Referral Web: combining social networks and collaborative filtering. In: Communications of the ACM 40 (1997), Nr. 3, S. 63-65
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