1. CSE 522 – Algorithmic and Economic Aspects of the Internet Instructors: Nicole Immorlica Mohammad Mahdian

2. Previously in this class Properties of social networks Probabilistic and game theoretic models for social networks

3. This Lecture Ranking Web Pages using Link Analysis

4. Components of a search engine Crawler  How to handle different types of URL  How often to crawl each page  How to detect “duplicates” Indexer  Data structures (to minimize # of disk access) Query handler  Find the set of pages that contain the query word.  Sort the results.

5. Sorting the search results HITS (Hypertext Induced Topic Selection) J. Kleinberg, “Authorative sources in a hyperlinked environment”, SODA 1998. PageRank S. Brin and L. Page, “The anatomy of a large-scale hypertextual web search engine”, WWW 1998. L. Page, S. Brin, R. Motwani, and Winograd, “The PageRank citation ranking: bringing order to the web”.

6. Difficulties Too many hits (“abundance”) # indexed pages: 110,000 in 94; 100,000,000 in 97. ) Often too many pages contain the query. Sometimes pages are not suff. self-descriptive. Brin & Page: As of Nov 97, only one in the top four commercial search engine finds itself! Need to find “popular” pages.

7. Link analysis Instead of using text analysis, we analyze the structure of hyperlinks to extract information about the popularity of a page. Advantages:  No need for complicated text analysis  Less manipulable, and independent of one person’s point of view. (think of it as a voting system).

8. Relevance vs. popularity Need to achieve a balance between relevance and popularity. Kleinberg’s approach: construct a focused subgraph based on relevance, and return the most popular page in this subgraph. Google’s approach: compute a measure of relevance (considering how many times and in what form [title/url/font size/anchor] the query appears in the page), and multiply with a popularity measure called PageRank.

9. Constructing a focused subgraph Desired properties:  Relatively small  Rich in relevant pages  Contains most of the strongest authorities on the subject.

10. Constructing a focused subgraph Given query , start with the set R  of the top ~200 text-based hits for . Add to this set:  the set of pages that have a link from a page in R  ;  the set of pages that have a link to a page p in R , with an upper limit of ~50 pages per p 2 R .ssdf  Call the resulting set S . Find the most “authorative” page in G[S  ].

11. Finding authorities Approach 1: vertices with the largest in- degrees This approach is used to evaluate scientific citations (the “impact factor”). Deficiencies:  A page might have a large in-degree from low- quality pages.  “universally popular” pages often dominate the result.  Easy to manipulate.

12. Finding authorities Approach 2: define the set of authorities recursively.  Best authorities on a subject have a large in- degree from the best hubs on the subject.  Best hubs on a subject give links to the best authorities on the subject. Formulation as a principal eigenvector

13. Discussion This algorithm can also be used to find the closest pages to a give page p.  Let R  be the set of at most ~200 pages that point to p. Can also compute multiple sets of hubs and authorities.

14. PageRank Again, the idea is a recursive definition of importance:  An important page is a page that has many links from other important pages. Problems:  Not always well-defined.  Pages with no out-degree form rank sinks.

15. PageRank Fix: consider a “random surfer”, which every time either clicks on a random link, or with probability , gets bored and starts again from a random page. PageRank takes  ¼ 1/7, and uses a non- uniform distribution for starting again.