1. Network Structure and Web Search Networked Life CIS 112 Spring 2010 Prof. Michael Kearns

2. Beyond Macroscopic Structure Broder et al. report on coarse overall structure of the web Use and construction of the web are more fine-grained –people browse the web for certain information or topics –people build pages that link to related or “similar” pages How do we quantify & analyze this more detailed structure? We’ll examine two related examples: –Kleinberg’s hubs and authorities automatic identification of “web communities” –PageRank automatic identification of “important” pages one of the main criteria used by Google –both rely mainly on the link structure of the web –both have an algorithm and a theory supporting them

3. Hubs and Authorities Suppose we have a large collection of pages on some topic –possibly the results of a standard web search Some of these pages are highly relevant, others not at all How could we automatically identify the important ones? What’s a good definition of importance? Kleinberg’s idea: there are two kinds of important pages: –authorities: highly relevant pages –hubs: pages that point to lots of relevant pages If you buy this definition, it further stands to reason that: –a good hub should point to lots of good authorities –a good authority should be pointed to by many good hubs –this logic is, of course, circular We need some math and an algorithm to sort it out

4. The HITS System (Hyperlink-Induced Topic Search) Given a user-supplied query Q: –assemble root set S of pages (e.g. first 200 pages by AltaVista) –grow S to base set T by adding all pages linked (undirected) to S –might bound number of links considered from each page in S Now consider directed subgraph induced on just pages in T For each page p in T, define its –hub weight h(p); initialize all to be 1 –authority weight a(p); initialize all to be 1 Repeat “forever”: –a(p) := sum of h(q) over all pages q  p –h(p) := sum of a(q) over all pages p  q –renormalize all the weights This algorithm will always converge! –weights computed related to eigenvectors of connectivity matrix –further substructure revealed by different eigenvectors Here are some examplesexamples

5. The PageRank Algorithm Let’s define a measure of page importance we will call the rank Notation: for any page p, let –N(p) be the number of forward links (pages p points to) –R(p) be the (to-be-defined) rank of p Idea: important pages distribute importance over their forward links So we might try defining –R(p) := sum of R(q)/N(q) over all pages q  p –can again define iterative algorithm for computing the R(p) –if it converges, solution again has an eigenvector interpretation –problem: cycles accumulate rank but never distribute it The fix: –R(p) := [sum of R(q)/N(q) over all pages q  p] + E(p) –E(p) is some external or exogenous measure of importance –some technical details omitted here (e.g. normalization) Let’s play with the PageRank calculatorPageRank calculator

6. A F D B C E G

7. The “Random Surfer” Model Let’s suppose that E(p) sums to 1 (normalized) Then the resulting PageRank solution R(p) will –also be normalized –can be interpreted as a probability distribution R(p) is the stationary distribution of the following process: –starting from some random page, just keep following random links –if stuck in a loop, jump to a random page drawn according to E(p) –so surfer periodically gets “bored” and jumps to a new page –E(p) can thus be personalized for each surfer An important component of Google’s search criteria

8. But What About Content? PageRank and Hubs & Authorities –both based purely on link structure –often applied to an pre-computed set of pages filtered for content So how do (say) search engines do this filtering? This is the domain of information retrieval

9. Basics of Information Retrieval Represent a document as a “bag of words”: –for each word in the English language, count number of occurences –so d[i] is the number of times the i-th word appears in the document –usually ignore common words (the, and, of, etc.) –usually do some stemming (e.g. “washed”  “wash”) –vectors are very long (~100Ks) but very sparse –need some special representation exploiting sparseness Note all that we ignore or throw away: –the order in which the words appear –the grammatical structure of sentences (parsing) –the sense in which a word is used firing a gun or firing an employee –and much, much more…

10. Bag of Words Document Comparison View documents as vectors in a very high-dimensional space Can now import geometry and linear algebra concepts Similarity between documents d and e: –  d[i]*e[i] over all words i –may normalize d and e first –this is their projection onto each other Improve by using TF/IDF weighting of words: –term frequency --- how frequent is the word in this document? –inverse document frequency --- how frequent in all documents? –give high weight to words with high TF and low IDF Search engines: –view the query as just another “document” –look for similar documents via above

11. Looking Ahead: Left Side vs. Right Side So far we are discussing the “left hand” search results on GoogleGoogle –a.k.a “organic” search; “Right hand” or “sponsored” search: paid advertisements in a formal market –We will spend a lecture on these markets later in the term Same two types of search/results on Yahoo!, MSN,… Common perception: –organic results are “objective”, based on content, importance, etc. –sponsored results are subjective advertisements But both sides are subject to “gaming” (strategic behavior)… –organic: invisible terms in the html, link farms and web spam, reverse engineering –sponsored: bidding behavior, “jamming” –optimization of each side has its own industry: SEO and SEMSEOSEM … and perhaps to outright fraud –organic: typo squattingtypo squatting –sponsored: click fraud More later…