PageRank-4

PageRank-4 - CS345 Data Mining Link Analysis Algorithms...

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CS345 Data Mining Link Analysis Algorithms Page Rank Anand Rajaraman, Jeffrey D. Ullman
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Link Analysis Algorithms b Page Rank b Hubs and Authorities b Topic-Specific Page Rank b Spam Detection Algorithms b Other interesting topics we won’t cover s Detecting duplicates and mirrors s Mining for communities
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Ranking web pages b Web pages are not equally “important” s www.joe-schmoe.com v www.stanford.edu b Inlinks as votes s www.stanford.edu has 23,400 inlinks s www.joe-schmoe.com has 1 inlink b Are all inlinks equal? s Recursive question!
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Simple recursive formulation b Each link’s vote is proportional to the importance of its source page b If page P with importance x has n outlinks, each link gets x/n votes b Page P ’s own importance is the sum of the votes on its inlinks
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Simple “flow” model The web in 1839 Yahoo M’soft Amazon y a m y/2 y/2 a/2 a/2 m y = y /2 + a /2 a = /2 + m = /2
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Solving the flow equations b 3 equations, 3 unknowns, no constants s No unique solution s All solutions equivalent modulo scale factor b Additional constraint forces uniqueness s y+a+m = 1 s y = 2/5, a = 2/5, m = 1/5 b Gaussian elimination method works for small examples, but we need a better method for large graphs
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Matrix formulation b Matrix M has one row and one column for each web page b Suppose page j has n outlinks s If j ! i, then M ij =1/n s Else M ij =0 b M is a column stochastic matrix s Columns sum to 1 b Suppose r is a vector with one entry per web page s r i is the importance score of page i s Call it the rank vector s | r | = 1
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Example Suppose page j links to 3 pages, including i i j M r r = i 1/3
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Eigenvector formulation b The flow equations can be written r = Mr b So the rank vector is an eigenvector of the stochastic web matrix s In fact, its first or principal eigenvector, with corresponding eigenvalue 1
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Example Yahoo M’soft Amazon y 1/2 1/2 0 a 1/2 0 1 m 0 1/2 0 y a m y = y /2 + a /2 a = /2 + m = /2 r = Mr y 1/2 1/2 0 y a = 1/2 0 1 a m 0 1/2 0 m
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Power Iteration method b Simple iterative scheme (aka relaxation ) b Suppose there are N web pages b Initialize: r 0 = [1/N,….,1/N] T b Iterate: r k+1 = Mr k b Stop when | r k+1 - r k | 1 < ε s | x | 1 = 1 i N |x i | is the L 1 norm s Can use any other vector norm e.g., Euclidean
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This document was uploaded on 03/04/2012.

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PageRank-4 - CS345 Data Mining Link Analysis Algorithms...

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