Active (Robot Assembled) Search Engines: Active search engines rely on computerized retrieval mechanisms. Referred to as "spiders", "crawlers", or "robots", these mechanisms visit Web sites and retrieve relevant keywords to index and store in a searchable database.

Advantages

• Revisit Web sites on a regular basis to look for changes or updates.
• Provide a ranking of search results based on relevancy.
• Useful for accessing hard-to-find information.

Disadvantages

• Rankings are sometimes questionable because they are computer-generated rather than human produced.
• Lack of human involvement can lead to poor quality control.

Examples

Alta Vista, HotBot, InfoSeek, Lycos, Excite

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Passive (Human-Selected) Search Engines

Passive search engines, often referred to as directories, are human controlled and do not roam the Web directly. They rely on individual submissions that are reviewed and indexed by subject category.

Advantages

• Provide users with a subject directory and a short description of each site.
• Human involvement usually produces a higher relevancy of results.

Disadvantages

• Contain far fewer records than active search engines.

Examples

Yahoo, LookSmart, Librarians' Index to the Internet

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Meta Search Engines

Meta search engines do not create their own databases. They rely on existing active and passive search engine indexes to retrieve search results.

Advantages

• Remove most duplication from search results.
• Save users time by searching several search engines at once.

Disadvantages

• Redundancies can occur in search results.
• Displayed results are often confusing.

Examples

Dogpile, Metacrawler, Webcrawler

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Specialized Search Engines

Specialized search engines offer additional features that active, passive and meta search engines do not provide. Examples:

Google

• Indexes Web sites based on how many other popular sites have linked to them.
• Valuable for finding Web sites in response to broad searches such as "cars" or "travel" because others have shown the retrieved sites are useful by linking to them.
• Learn more about using Google Scholar at Cornell.

Ask Jeeves

• Human-generated search engine.
• Allows users to pose search requests in the form of questions rather than keywords.
• Attempts to direct users to the exact page or pages that answers the question.
• Has a back-up metacrawler component that will search other indexes if a match is not found within its own database.

Northern Light

• Provides users with a set of special collection documents that are not readily available to search engine spiders. These documents are collected from a variety of sources, including newswires, magazines and other databases.
• Searching these documents is free, however there is a charge to view them.

Rollyo

• Allows you to create a list of sites that the search engine willl search.
• Useful when you want to limit your search to a number of trusted sites.

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