Google PageRank

Explanation of Google's PageRank algorithm with some examples followed by a discussion of related Search engine optimisation (SEO) issues.

Transcript

1. Vrije Universiteit Brussel, August 25, 2008 Google PageRank Beat Signer Global Information Systems Research Group Department of Computer Science ETH Zurich
2. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Overview ▪ History of PageRank ▪ PageRank algorithm ▪ Examples ▪ Implications for website development 2
3. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch History of PageRank ▪ Developed as part of an academic project at Stanford University ▪ research platform to aid understanding of large-scale web data and enable researches to easily experiment with new search technologies ▪ Larry Page and Sergey Brin worked on the project about a new kind of search engine (1995-1998) which finally led to a functional prototype called Google 3 Larry Page Sergey Brin
4. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Web Search Until 1998 ▪ Find all documents using a query term ▪ use information retrieval (IR) solutions ▪ ranking based on "on the page factors" → problem: poor quality of search results (order) ▪ Page and Brin proposed to compute the absolute qualtity of a page (PageRank) ▪ based on the number and quality of pages linking to a page (votes) 4
5. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch PageRank ▪ A page has a high PageRank R if ▪ there are many pages linking to it ▪ or, if there are some pages with a high PageRank linking to it ▪ Total score = IR score x PageRank 5 P1 R1 P2 R2 P3 R3 P4 R4 P5 R5 P6 R6 P7 R7 P8 R8
6. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch PageRank Algorithm ▪ where ▪ Bi is the set of pages that link to page Pi ▪ Lj is the number of outgoing links for page Pj 6   = i j B P j j i L P R P R ) ( ) ( P1 P2 P3 P1 1 P2 1 P3 1 P1 1.5 P2 1.5 P3 0.75 P1 1.5 P2 1.5 P3 0.75
7. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Matrix Representation ▪ Let us define a hyperlink matrix H 7 P1 P2 P3     = otherwise 0 if 1 i j j ij B P L H           = 0 2 1 0 0 0 1 1 2 1 0 H ( )   i P R = R and HR R = R is an eigenvector of H with eigenvalue 1 →
8. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Matrix Representation ... ▪ We can use the power method to find R 8 t t HR R = +1           = 0 2 1 0 0 0 1 1 2 1 0 H For our example this results in or   1 2 2 = R   2 . 0 4 . 0 4 . 0
9. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Dangling Pages ▪ Problem with pages that have no outbound links (P2 ) 9 P1 P2       = 0 1 0 0 H and   0 0 = R       = 2 1 0 2 1 0 C       = + = 2 1 1 2 1 0 C H S and C C
10. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Strongly Connected Pages (Graph) ▪ Add new transition probabilities between all pages ▪ with probability d we follow the hyperlink structure S ▪ with probability 1-d we choose a random page 10 P1 P2 P3 P4 P5 ( ) S 1 G d n d + − = 1 1 GR R = 1-d 1-d 1-d
11. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Examples 11 ( ) S 1 G d n d + − = 1 1 A1 0.26 A2 0.37 A3 0.37
12. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Examples ... 12 A1 0.13 A2 0.185 A3 0.185 B1 0.13 B2 0.185 B3 0.185 ( ) 5 . 0 = A P ( ) 5 . 0 = B P ( ) S 1 G d n d + − = 1 1
13. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Examples ... 13 A1 0.10 A2 0.14 A3 0.14 B1 0.22 B2 0.20 B3 0.20 ( ) 38 . 0 = A P ( ) 62 . 0 = B P ( ) S 1 G d n d + − = 1 1
14. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Examples ... 14 A1 0.3 A2 0.23 A3 0.18 B1 0.10 B2 0.095 B3 0.095 ( ) 71 . 0 = A P ( ) 29 . 0 = B P ( ) S 1 G d n d + − = 1 1
15. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Examples ... 15 A1 0.35 A2 0.24 A3 0.18 B1 0.09 B2 0.07 B3 0.07 ( ) 77 . 0 = A P ( ) 23 . 0 = B P ( ) S 1 G d n d + − = 1 1
16. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Examples ... 16 A1 0.33 A2 0.17 A3 0.175 B1 0.08 B2 0.06 B3 0.06 ( ) 86 . 0 = A P ( ) 14 . 0 = B P A4 0.125 ( ) S 1 G d n d + − = 1 1
17. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Implications for Website Development ▪ First make sure that your page gets indexed ▪ "on the page factors" ▪ Think about your site's internal link structure ▪ create many internal links for important pages ▪ be "careful" about where to put outgoing links ▪ Increase the number of pages ▪ Ensure that webpages are addressed consistently ▪ http://www.vub.ac.be  http://www.vub.ac.be/index.php ▪ Make sure that you get links from good websites 17
18. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Consistent Addressing of Webpages 18
19. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Other Search Engine Optimisations (SEO) ▪ Internet marketing has become big business ▪ white hat and black hat optimisations ▪ Black hat optimisations ▪ link farms ▪ spamdexing in guestbooks etc. <a rel="nofollow" href="…">…</a> ▪ selling/buying links ▪ … ▪ Is PageRank fair? 19
20. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch Conclusions ▪ PageRank algorithm ▪ absolute quality of a page based on incoming links ▪ random surfer model ▪ computed as eigenvector of Google matrix G ▪ Implications for website development and SEO ▪ PageRank is just one (important) factor 20
21. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch References ▪ The PageRank Citation Ranking: Bringing Order to the Web, L. Page, S. Brin, R. Motwani and T. Winograd, January 1998 ▪ The Anatomy of a Large-Scale Hypertextual Web Search Engine, S. Brin and L. Page, Computer Networks and ISDN Systems, 30(1-7), April 1998
22. Vrije Universiteit Brussel, August 25, 2008 Beat Signer, signer@inf.ethz.ch References … ▪ PageRank Uncovered, C. Ridings and M. Shishigin, September 2002 ▪ PageRank Calculator, http://www.webworkshop.net/pagerank_ calculator.php 22