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Focused Crawling A New Approach to Topic-Specific Web Resource Discovery Soumen Chakrabarti Martin van Den Berg Byron Dom.

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Presentation on theme: "Focused Crawling A New Approach to Topic-Specific Web Resource Discovery Soumen Chakrabarti Martin van Den Berg Byron Dom."— Presentation transcript:

1 Focused Crawling A New Approach to Topic-Specific Web Resource Discovery Soumen Chakrabarti Martin van Den Berg Byron Dom

2 WWW Portals and portholes  Popular search portals and directories  Useful for generic needs  Difficult to do serious research  Information needs of net-savvy users are getting very sophisticated  Relatively little business incentive  Need handmade specialty sites: portholes  Resource discovery must be personalized

3 WWW Quote The emergence of portholes will be one of the major Internet trends of As people become more savvy users of the Net, they want things which are better focused on meeting their specific needs. We're going to see a whole lot more of this, and it's going to potentially erode the user base of some of the big portals. Jim Hake (Founder, Global Information Infrastructure Awards)

4 WWW Scenario  Disk drive research group wants to track magnetic surface technologies  Compiler research group wants to trawl the web for graduate student resumés  ____ wants to enhance his/her collection of bookmarks about ____ with prominent and relevant links  Virtual libraries like the Open Directory Project and the Mining Co.Open Directory ProjectMining Co.

5 WWW Structured web queries  How many links were found from an environment protection agency site to a site about oil and natural gas in the last year?  Apart from cycling, what is the most common topic cited by pages on cycling?  Find Web research pages which are widely cited by Hawaiian vacation pages

6 WWW Goal  Automatically construct a focused portal (porthole) containing resources that are  Relevant to the user’s focus of interest  Of high influence and quality  Collectively comprehensive  Answer structured web queries by selectively exploring the topics involved in the query

7 WWW Tools at hand  Keyword search engines  Synonymy, polysemy  Abundance, lack of quality  Hand compiled topic directories  Labor intensive, subjective judgements  Resources automatically located using keyword search and link graph distillation  Dependence on large crawls and indices

8 WWW Estimating popularity  Extensive research on social network theory  Wasserman and Faust  Hyperlink based  Large in-degree indicates popularity/authority  Not all votes are worth the same  Several similar ideas and refinements  Googol (Page and Brin) and HITS (Kleinberg)  Resource compilation (Chakrabarti et al)  Topic distillation (Bharat and Henzinger)

9 WWW Topic distillation overview  Given web graph and query  Search engine selects sub-graph  Expansion, pruning and edge weights  Nodes iteratively transfer authority to cited neighbors Search Engine Query The Web Selected subgraph

10 WWW Preliminary distillation-based approach  Design a keyword query to represent a topic  Run topic distillation periodically  Refine query through trial-and-error  Works well if answer is partially known, e.g., European airlines  +swissair +iberia +klm

11 WWW

12 WWW Problems with preliminary approach  Dependence on large web crawl and index  System = crawler + index + distiller  Unreliability of keyword match  Engines differ significantly on a given query due to small overlap [Bharat and Bröder]  Narrow, arbitrary view of relevant subgraph  Topic model does not improve over time  Difficulty of query construction  Lack of output sensitivity

13 WWW Query construction +“power suppl*” “switch* mode” smps -multiprocessor* “uninterrupt* power suppl*” ups -parcel* /Companies/Electronics/Power_Supply

14 WWW Query complexity  Complex queries (966 trials)  Average words 7.03  Average operators ( +*–" ) 4.34  Typical Alta Vista queries are much simpler [Silverstein, Henzinger, Marais and Moricz]  Average query words 2.35  Average operators ( +*–" ) 0.41  Forcibly adding a hub or authority node helped in 86% of the queries

15 WWW Query complexity  Complex queries needed for distillation  Typical Alta Vista queries are much simpler (Silverstein, Henzinger, Marais and Moricz)  Forcing a hub or authority helps 86% of the time

16 WWW Output sensitivity  Say the goal is to find a comprehensive collection of recreational and competitive bicycling sites and pages  Ideally effort should scale with size of the result  Time spent crawling and indexing sites unrelated to the topic is wasted  Likewise, time that does not improve comprehensiveness is wasted

17 WWW Proposed solution  Resource discovery system that can be customized to crawl for any topic by giving examples  Hypertext mining algorithms learn to recognize pages and sites about the given topic, and a measure of their centrality  Crawler has guidance hooks controlled by these two scores

18 WWW Administration scenario Taxonomy Editor Current Examples Suggested Additional Examples Drag

19 WWW Relevance All Bus&EconRecreation CompaniesCycling Bike Shops Mt.Biking Clubs Arts... Path nodes Good nodes Subsumed nodes

20 WWW Classification  How relevant is a document w.r.t. a class?  Supervised learning, filtering, classification, categorization  Many types of classifiers  Bayesian, nearest neighbor, rule-based  Hypertext  Both text and links are class-dependent clues  How to model link-based features?

21 WWW The “bag-of-words” document model  Decide topic; topic c is picked with prior probability  (c);  c  (c) = 1  Each c has parameters  (c,t) for terms t  Coin with face probabilities  t  (c,t) = 1  Fix document length and keep tossing coin  Given c, probability of document is

22 WWW Exploiting link features  c=class, t=text, N=neighbors  Text-only model: Pr[t|c]  Using neighbors’ text to judge my topic: Pr[t, t(N) | c]  Better model: Pr[t, c(N) | c]  Non-linear relaxation ?

23 WWW Improvement using link features  9600 patents from 12 classes marked by USPTO  Patents have text and cite other patents  Expand test patent to include neighborhood  ‘Forget’ fraction of neighbors’ classes

24 WWW Putting it together Taxonomy Database Taxonomy Editor Example Browser Crawl Database Hypertext Classifier (Learn) Topic Models Hypertext Classifier (Apply) Scheduler Workers Topic Distiller Feedback

25 WWW Monitoring the crawler Time Relevance One URL Moving Average

26 WWW Measures of success  Harvest rate  What fraction of crawled pages are relevant  Robustness across seed sets  Separate crawls with random disjoint samples  Measure overlap in URLs and servers crawled  Measure agreement in best-rated resources  Evidence of non-trivial work  #Links from start set to the best resources

27 WWW Harvest rate UnfocusedFocused

28 WWW Crawl robustness URL OverlapServer Overlap Crawl 1 Crawl 2

29 WWW Top resources after one hour  Recreational and competitive cycling     HIV/AIDS research and treatment     Purer and better than root set

30 WWW

31 WWW

32 WWW Distance to best resources Cycling: cooperativeMutual funds: competitive

33 WWW Robustness of resource discovery  Sample disjoint sets of starting URL’s  Two separate crawls  Find best authorities  Order by rank  Find overlap in the top-rated resources

34 WWW Related work  WebWatcher, HotList and ColdList  Filtering as post-processing, not acquisition  ReferralWeb  Social network on the Web  Ahoy!, Cora  Hand-crafted to find home pages and papers  WebCrawler, Fish, Shark, Fetuccino, agents  Crawler guided by query keyword matches

35 WWW Comparison with agents  Agents usually look for keywords and hand-crafted patterns  Cannot learn new vocabulary dynamically  Do not use distance-2 centrality information  Client-side assistant  We use taxonomy with statistical topic models  Models can evolve as crawl proceeds  Combine relevance and centrality  Broader scope: inter- community linkage analysis and querying

36 WWW Conclusion  New architecture for example-driven topic- specific web resource discovery  No dependence on full web crawl and index  Modest desktop hardware adequate  Variable radius goal-directed crawling  High harvest rate  High quality resources found far from keyword query response nodes

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