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Motivation

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PageRank Algorithm used by Google inc

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Motivation

1. 1. Motivation 1. Link algorithm(SALSA) 2. Page Rank Algorithm
2. 2. SEARCH PHRASE• Keyword Density• Accentuation within a document • HTML Tags• Not resistant against automatically generated web
4. 4. Page rank algorithmPageRank is an expected value for the random surfer visiting a page, when he restarts this procedure as often as the web has pages • Comparision of pages • The higher the better • Recursive Determination
5. 5. introductionSergey Brin Larry Page
6. 6. algorithmPageRank does not rank web sites as a whole, but is determined for each page individually *This means that the more outbound links a page T has, the less will page A benefit from a link to it on page T.
7. 7. The random surfer model • The probability that the random surfer clicks on one link is solely given by the number of links on that page • The surfer does not click on an infinite number of links, but gets bored sometimes and jumps to another page at random. damping factor ‘d’
8. 8. Differentnotation
9. 9. characteristic A b cPR(A) = 0.5 + 0.5 PR(C) PR(A) = 14/13 = 1.07692308PR(B) = 0.5 + 0.5 (PR(A) / 2) PR(B) = 10/13 = 0.76923077PR(C) = 0.5 + 0.5 (PR(A) / 2 + PR(B)) PR(C) = 15/13 = 1.15384615
10. 10. The iterativeIteration PR(A) PR(B) PR(C) 0 1 1 1 1 1 0.75 1.125 2 1.0625 0.765625 1.1484375 3 1.07421875 0.76855469 1.15283203 4 1.07641602 0.76910400 1.15365601 5 1.07682800 0.76920700 1.15381050 6 1.07690525 0.76922631 1.15383947 7 1.07691973 0.76922993 1.15384490 8 1.07692245 0.76923061 1.15384592 9 1.07692296 0.76923074 1.15384611 10 1.07692305 0.76923076 1.15384615 11 1.07692307 0.76923077 1.15384615
11. 11. Implementation ingoogle search engine • Page specific factors • Body text • Content of title tag • URL of document • Anchor text of inbound links • Page rank Page Inbound IRfactors links Score * IR score is multiplied with the Page Rank
12. 12. Effect of inbound links• d × PR(X) / C(X) where PR(X) is the PageRank of page X and C(X) is the total number ofits outbound links. But page A usually links to other pages itself. Thus, these pagesget a PageRank benefit also. If these pages link back to page A, page A will have aneven higher PageRank benefit from its additional inbound link• Influence of Damping Factor
13. 13. • Initially all have Pagerank 1 • We presume a constant Pagerank PR(X) of 10 • Damping factor is equal to 0.5 PR(A)= 0.5 + 0.5 (PR(X) + PR(D)) = 5.5 + 0.5 PR(D) PR(B)= 0.5 + 0.5 PR(A) PR(C)= 0.5 + 0.5 PR(B) PR(D)= 0.5 + 0.5 PR(C) PR(A) = 19/3 = 6.33 PR(B) = 11/3 = 3.67 PR(C) = 7/3 = 2.33 d × PR(X) / C(X) = 0,5 × 10 / 1 = 5 PR(D) = 5/3 = 1.67* The higher the damping factor, the larger is the effect of an additional inbound link forthe PageRank of the page that receives the link and the more evenly distributesPageRank over the other pages of a site.
14. 14. Effect of outbound linksPR(A) = 0.25 + 0.75 PR(B)PR(B) = 0.25 + 0.375 PR(A)PR(C) = 0.25 + 0.75 PR(D) + 0.375 PR(A)PR(D) = 0.25 + 0.75 PR(C) PR(A) = 14/23 PR(C) = 35/23 PR(B) = 11/23 PR(D) = 32/23*Adding a link has no effect on the total PageRank of the web. Additionally, thePageRank benefit for one site equals the PageRank loss of the other.
15. 15. Dangling links PR(A) = 0.25 + 0.75 PR(B) PR(B) = 0.25 + 0.375 PR(A) PR(C) = 0.25 + 0.375 PR(A) PR(A) = 14/23 PR(B) = 11/23 PR(C) = 11/23Dangling links could have major impacts on PageRank.
16. 16. • In order to prevent PageRank from the negative effects of dangling links, pages wihout outbound links have to be removed from the database until the PageRank values are computed. • According to Page and Brin, the number of outbound links on pages with dangling links is thereby normalised PR(C) = 0.25 + 0.375 PR(A) = 0.625*The accumulated PageRank does not equal the number of pages, but at leastall pages which have outbound links are not harmed from the danging links
17. 17. Effect of the number of pages PR(A) = 260/14 PR(A) = 266/14 PR(A) = 13.97 PR(A) = 11.97 PR(B) = 101/14 PR(B) = 70/14 PR(B) = 10.73 PR(B) = 9.23 PR(C) = 101/14 PR(C) = 70/14 PR(C) = 8.30 PR(C) = 7.17 PR(D) = 70/14 PR(D) = 5.63 * The PageRank algorithm tends to privilege smaller web sites.
18. 18. The distribution of pagerank for s.e.o. PR(A) = 8 PR(A) = 7 PR(B) = 2.5 PR(B) = 3 PR(C) = 2.5 PR(C) = 3PageRank will distribute for the purpose of search engine optimisation more equallyamong the pages of a site, the more the hierarchically lower pages are interlinked.
19. 19. Concentration of outbound links PR(A) = 1 PR(A) = 17/13 PR(B) = 2/3 PR(B) = 28/39 PR(C) = 2/3 PR(C) = 28/39 PR(D) = 2/3 PR(D) = 28/39Concentrate external outbound links on as few pages as possible, as long as it does not lessen asites usabilty.
20. 20. Link exchanges PR(A) = 4/3 PR(D) = 4/3 PR(A) = 3/2 PR(D) = 3/2 PR(B) = 5/6 PR(E) = 5/6 PR(B) = 3/4 PR(E) = 3/4 PR(C) = 5/6 PR(F) = 5/6 PR(C) = 3/4 PR(F) = 3/4A link exchange is thus advisable, if one page (e.g. the rootpage of a site) shall be optimised for one important key