1 Chapter 5 (3 rd ed) Your library is an excellent resource tool. Your library is an excellent resource tool

2 Finding information on the Web But how do we ever find anything online? We may have a URL to get us started e.g. and from there we follow the links We may have a URL to get us started e.g. and from there we follow the links Or, we may use a search engine Or, we may use a search engine

3 Search Engines A search engine is a collection of computer programs that helps us find information on the World Wide Web A search engine is a collection of computer programs that helps us find information on the World Wide Web Some are: Google, Yahoo! Search, Ask.com, AlltheWeb, Lycos, AltaVista, Exalead, Bing.com Some are: Google, Yahoo! Search, Ask.com, AlltheWeb, Lycos, AltaVista, Exalead, Bing.com

4 Search Engines A search engine has 2 basic parts: Crawler Crawler Query Processor Query Processor

5 Crawler Also called a “bot” or “spider” Also called a “bot” or “spider” “Crawls” around the Web looking for new Web pages to add to its search engine’s index (just a database) “Crawls” around the Web looking for new Web pages to add to its search engine’s index (just a database) Records keywords, images from a new page in its index Records keywords, images from a new page in its index Keywords can come from the title, body or anchor Keywords can come from the title, body or anchor Uses links on a page to find further pages to visit Uses links on a page to find further pages to visit

6 Crawler Most search engines will also let Web authors submit their pages to be crawled. Most search engines will also let Web authors submit their pages to be crawled. Each search engine has its own crawler, thus its own index (or database of crawled pages). Each search engine has its own crawler, thus its own index (or database of crawled pages).

7 Really? NO search engine has EVERY Web page indexed. NO search engine has EVERY Web page indexed. Less than half of the searchable Web is fully searchable in Google. Less than half of the searchable Web is fully searchable in Google. Approximately half of the pages in any search engine’s index exist only in that index. Approximately half of the pages in any search engine’s index exist only in that index.

8 Query Processor We interact with the query processor. We interact with the query processor. It takes the keywords we enter ( called a query) and looks for them in the index. It takes the keywords we enter ( called a query) and looks for them in the index. Query processors do NOT search the World Wide Web when you submit a query. Query processors do NOT search the World Wide Web when you submit a query.

9 Queries Query: bird flu Words separated by spaces treated as two separate words by most search engines Words separated by spaces treated as two separate words by most search engines Same as bird AND flu Same as bird AND flu i.e. finds pages with both words anywhere and in any order i.e. finds pages with both words anywhere and in any order

10 Queries with a Phrase Query: “bird flu” Finds pages with this exact phrase Finds pages with this exact phrase

11 Don’t Believe Everything You Find Online Check out what you find at one site by looking for the same information at other sites. Check out what you find at one site by looking for the same information at other sites.