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E NHANCING M ATRIX F ACTORIZATION T HROUGH I NITIALIZATION FOR I MPLICIT F EEDBACK D ATABASES Balázs Hidasi Domonkos Tikk Gravity R&D Ltd. Budapest University of Technology and Economics C A RR WORKSHOP, 14 TH F EBRUARY 2012, L ISBON

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O UTLINE Matrix factoriztion Initialization concept Methods Naive SimFactor Results Discussion 2/19

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M ATRIX F ACTORIZATION Collaborative Filtering One of the most common approaches Approximates the rating matrix as product of low-rank matrices R Q P Items Users 3/19

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M ATRIX FACTORIZATION Initialize P and Q with small random numbers Teach P and Q Alternating Least Squares Gradient Descent Etc. Transforms the data to a feature space Separately for users and items Noise reduction Compression Generalization 4/19

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I MPLICIT FEEDBACK No ratings User-item interactions (events) Much noisier Presence of an event might not be positive feedback Absence of an event does not mean negative feedback No negative feedback is available! More common problem MF for implicit feedback Less accurate results due to noise Mostly ALS is used Scalability problems (rating matrix is dense) 5/19

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C ONCEPT Good MF model The feature vectors of similar entities are similar If data is too noisy similar entities wont be similar by their features Start MF from a good point Feature vector similarities are OK Data is more than just events Metadata Info about items/users Contextual data In what context did the event occured Can we incorporate those to help implicit MF? 6/19

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N AIVE APPROACH Describe items using any data we have (detailed later) Long, sparse vectors for item description Compress these vectors to dense feature vectors PCA, MLP, MF, … Length of desired vectors = Number of features in MF Use these features as starting points 7/19

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N AIVE APPROACH Compression and also noise reduction Does not really care about similarities But often feature similarities are not that bad If MF is used Half of the results is thrown out Description of items Descriptor features Item features Descriptor s Items 8/19

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S IM F ACTOR ALGORITHM Try to preserve similarities better Starting from an MF of item description Similarities of items: DD Some metrics require transformation on D Description of items (D) Descriptors features Item features Descriptor s Items Description of items (D) Description of items (D) Item similaritie s (S) = 9/19

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S IM F ACTOR ALGORITHM Similarity approximation YY KxK symmetric Eigendecomposition Item similaritie s (S) Descriptors features (Y) Item features (X) Descriptors features (Y) Item features (X) Item similaritie s (S) Item features (X) YY 10/19

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S IM F ACTOR ALGORITHM λ diagonal λ = SQRT(λ) * SQRT(λ) X*U*SQRT(λ) = (SQRT(λ)*U*X)=F F is MxK matrix S F * F F used for initialization YY U SQRT (λ) U Item similaritie s (S) Item features (X) SQRT (λ) Item similaritie s (S) F F UU λ = 11/19

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C REATING THE DESCRIPTION MATRIX Any data about the entity Vector-space reprezentation For Items : Metadata vector (title, category, description, etc) Event vector (who bought the item) Context-state vector (in which context state was it bought) Context-event (in which context state who bought it) For Users : All above except metadata Currently: Choose one source for D matrix Context used: seasonality 12/19

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E XPERIMENTS : S IMILARITY PRESERVATION Real life dataset: online grocery shopping events SimFactor approximates similarities better 13/19

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E XPERIMENTS : I NITIALIZATION Using different description matrices And both naive and SimFactor initialization Baseline: random init Evaluation metric: recall@50 14/19

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E XPERIMENTS : G ROCERY DB Up to 6% improvement Best methods use SimFactor and user context data 15/19

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E XPERIMENTS : I MPLICITIZED M OVIE L ENS Keeping 5 star ratings implicit events Up to 10% improvement Best methods use SimFactor and item context data 16/19

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D ISCUSSION OF RESULTS SimFactor yields better results than naive Context information yields better results than other descriptions Context information separates well between entities Grocery: User context Peoples routines Different types of shoppings in different times MovieLens: Item context Different types of movies watched on different hours Context-based similarity 17/19

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W HY CONTEXT ? Grocery example Correlation between context states by users low Why can context aware algorithms be efficient? Different recommendations in different context states Context differentiates well between entities Easier subtasks ITEM MonTueWedThuFriSatSun Mon1,000,79 0,780,760,700,74 Tue0,791,000,790,780,760,690,73 Wed0,79 1,000,790,760,700,74 Thu0,78 0,791,000,760,710,74 Fri0,76 1,000,710,72 Sat0,700,690,700,71 1,000,71 Sun0,740,730,74 0,720,711,00 USER MonTueWedThuFriSatSun Mon1,000,360,34 0,350,290,19 Tue0,361,000,34 0,330,290,19 Wed0,34 1,000,360,350,270,17 Thu0,34 0,361,000,390,300,16 Fri0,350,330,350,391,000,320,16 Sat0,29 0,270,300,321,000,33 Sun0,19 0,170,16 0,331,00 18/19

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C ONCLUSION & F UTURE WORK SimFactor Similarity preserving compression Similarity based MF initialization: Description matrix from any data Apply SimFactor Use output as initial features for MF Context differentitates between entities well Future work: Mixed description matrix (multiple data sources) Multiple description matrix Using different context information Using different similarity metrics 19/19

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T HANKS FOR YOUR ATTENTION ! For more of my recommender systems related research visit my website: http://www.hidasi.eu http://www.hidasi.eu

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