Download Search Engines

Search Engines
What Are They?
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Four Components
A database of references to webpages
 An indexing robot that crawls the WWW
 An interface
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Enables users to submit queries
 Displays results
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Information retrieval system
Each is unique, but are mostly the same
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Database
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Where user's query is matched
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Contains only essential parts of pages
Only includes pages that were indexed
Search engines are always out of date
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Web Crawler
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A robot that follows links
Records data it finds
Words in the webpage
 Metadata
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ALT attributes in IMG tags
Robot Exclusion Protocol
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Search Engine Interfaces
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Gathers input from users
Presents results from the IR system
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Often in ranked order
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Search Engine Interfaces
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Input
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User requirements
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Search expression, search limits
Presentation style
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Presentation format , search type
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Search Engine Interfaces
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Output
Results
 Descriptions
 Clusters
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Search Term Matching
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Trying to find a match in the database
Two main methods
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Keyword searching
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Matching single terms, computing cosine
Concept-based searching
Examining clusters of words
 Attempt to determine meaning of query and find
records related to that meaning
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Basic IR Features
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Boolean operators
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Extended operators
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AND, OR, NOT, grouping
NEAR, ADJACENT, (")
Stop word deletion
Stemming
Searching in fields (e.g. host)
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Ranked Output
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Most SEs produce ranked lists by applying
simple rules:
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Early words are more important
Title is very important
Frequency of occurrence matters for some
Infrequent words matter more
Modification date
Google is different:
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PageRankTM method based on popularity
Links as money
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Googlebombing
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Google spoofed from the lecture list
first hit from 1992
 Official GoogleBlog explanation
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What about the Invisible Web?
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Also known as the Deep Web
Documents that are on the WWW but
not indexed by Search Engines
Some are available only by submitting
forms
 Some are not generally accessible (in
subnets)
 Some are not in (X)HTML format
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The Invisible Web Isn't So
Invisible Anymore…
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More search engines parse non(X)HTML now than before
Because of awareness of the problem
companies are making more content
available using
Stable URLs
 Robot-friendly sitemaps
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But much content is still not indexed
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But, there's still plenty of
important yet invisible docs
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How to find them?
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Use database tools from the U.'s library
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Many of them are in databases
No one search engine covers everything
Especially for research articles
Use multiple search engines or a metacrawler
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dogpile is the most famous
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Search Engines
A Summary of Practical Advice
How To Succeed With SEs
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As a surfer:
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If you don't know what you are looking for
Use multiple SEs, or a meta-crawler
 Search within results
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If you don't know what you are looking for
Use multiple SEs, or a meta-crawler
 Use Boolean expressions or search within
results
 Consider specialized engines
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How To Succeed With SEs
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As a creator:
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HTML level
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Always use ALT attributes with <IMG>, etc.
Avoid frames
Make it easier to index
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Don't expect SEs to find your pages
Make links between your pages
Use metadata
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Informal: <meta name="description" …>
Formal: Dublin core and others
Increase your pages popularity
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Don’t use systematic reciprocal linking: rings, exchanges, lists
Page Rank™ is inversely proportional to outdegree
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How To Succeed With SEs
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As a creator (cont.)
For surfers:
Use <meta name="description" …>
 Don't expect surfers to start at top of your
hierarchy
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Don't rely on a hierarchy
 Include a context map near the top of each page
 Don't use frames
 Think through dynamic content implications
 Stickiness… is for another day
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