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Similarity & Recommendation - CWI Scientific Meeting - Sep 27th, 2013

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Recommender systems aim to predict the content that a user would like based on observations of the online behaviour of its users. Research in the Information Access group addresses different aspects of this problem, varying from how to measure recommendation results, how recommender systems relate to information retrieval models, and how to build effective recommender systems (note: last Friday, we won the ACM RecSys 2013 News Recommender Systems challenge). We would like to develop a general methodology to diagnose weaknesses and strengths of recommender systems. In this talk, I discuss the initial results of an analysis of the core component of collaborative filtering recommenders: the similarity metric used to find the most similar users (neighbours) that will provide the basis for the recommendation to be made. The purpose is to shed light on the question why certain user similarity metrics have been found to perform better than others. We have studied statistics computed over the distance distribution in the neighbourhood as well as properties of the nearest neighbour graph. The features identified correlate strongly with measured prediction performance - however, we have not yet discovered how to deploy this knowledge to actually improve recommendations made.

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Similarity & Recommendation - CWI Scientific Meeting - Sep 27th, 2013

  1. 1. Similarity & Recommendation Arjen P. de Vries arjen@cwi.nl CWI Scientific Meeting September 27th 2013
  2. 2. Recommendation • Informally: – Search for information “without a query” • Three types: – Content-based recommendation – Collaborative filtering (CF) • Memory-based • Model-based – Hybrid approaches
  3. 3. Recommendation • Informally: – Search for information “without a query” • Three types: – Content-based recommendation – Collaborative filtering • Memory-based • Model-based – Hybrid approaches Today’s focus!
  4. 4. Collaborative Filtering • Collaborative filtering (originally introduced by Patti Maes as “social information filtering”) 1. Compare user judgments 2. Recommend differences between similar users • Leading principle: People’s tastes are not randomly distributed –A.k.a. “You are what you buy”
  5. 5. Collaborative Filtering • Benefits over content-based approach – Overcomes problems with finding suitable features to represent e.g. art, music – Serendipity – Implicit mechanism for qualitative aspects like style • Problems: large groups, broad domains
  6. 6. Context • Recommender systems – Users interact (rate, purchase, click) with items
  7. 7. Context • Recommender systems – Users interact (rate, purchase, click) with items
  8. 8. Context • Recommender systems – Users interact (rate, purchase, click) with items
  9. 9. Context • Recommender systems – Users interact (rate, purchase, click) with items
  10. 10. Context • Nearest-neighbour recommendation methods – The item prediction is based on “similar” users
  11. 11. Context • Nearest-neighbour recommendation methods – The item prediction is based on “similar” users
  12. 12. Similarity
  13. 13. Similarity
  14. 14. Similarity s( , ) sim( , )s( , )
  15. 15. Research Question • How does the choice of similarity measure determine the quality of the recommendations?
  16. 16. Sparseness • Too many items exist, so many ratings will be missing • A user’s neighborhood is likely to extend to include “not-so-similar” users and/or items
  17. 17. “Best” similarity? • Consider cosine similarity vs. Pearson similarity • Most existing studies report Pearson correlation to lead to superior recommendation accuracy
  18. 18. “Best” similarity? • Common variations to deal with sparse observations: – Item selection: • Compare full profiles, or only on overlap – Imputation: • Impute default value for unrated items – Filtering: • Threshold on minimal similarity value
  19. 19. “Best” similarity? • Cosine superior (!), but not for all settings – No consistent results
  20. 20. Analysis
  21. 21. Distance Distribution • In high dimensions, nearest neighbour is unstable: If the distance from query point to most data points is less than (1 + ε) times the distance from the query point to its nearest neighbour Beyer et al. When is “nearest neighbour” meaningful? ICDT 1999
  22. 22. Distance Distribution Beyer et al. When is “nearest neighbour” meaningful? ICDT 1999
  23. 23. Distance Distribution • Quality q(n, f): Fraction of users for which the similarity function has ranked at least n percent of the user community within a factor f of the nearest neighbour’s similarity value (well... its corresponding distance)
  24. 24. Distance Distribution
  25. 25. NNk Graph • Graph associated with the top k nearest neighbours • Analysis focusing on the binary relation of whether a user does or does not belong to a neighbourhood – Ignore similarity values (already included in the distance distribution analysis)
  26. 26. NNk Graph
  27. 27. MRR vs. Features • Quality: – If most of the user population is far away, high similarity correlates with effectiveness – If most of the user population is close, high similarity correlates with ineffectiveness
  28. 28. MRR vs. Features
  29. 29. Conclusions (so far) • “Similarity features” correlate with recommendation effectiveness – “Stability” of a metric (as defined in database literature on k-NN search in high dimensions) is related to its ability to discriminate between good and bad neighbours
  30. 30. Future Work • How to exploit this knowledge to now improve recommendation systems?
  31. 31. News Recommendation Challenge
  32. 32. Thanks • Alejandro Bellogín – ERCIM fellow in the Information Access group Details: Bellogín and De Vries, ICTIR 2013.

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