Pearson correlation a Weight between users a and u Compute similarity matrix between users Use Pearson Correlation(-1, 0, 1) Let items be all items that users rated Pearson correlation (ai-rari-ru) ser AbC items Items ∑ ru) B|11-110 Pearson Correlation ◼ Weight between users a and u ◼ Compute similarity matrix between users ◼ Use Pearson Correlation (-1, 0, 1) ◼ Let items be all items that users rated   −  − − − = items u u i items a a i items u u i a a i a u r r r r r r r r items w 2 , 2 , , , , ( ) ( ) ( )( ) | | 1 Pearson Correlation A B C A 1 1 -1 B 1 1 -1 C -1 -1 1 User User