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Lesson 5: Specialized Search Engines and Subject Directories
For many research needs, the general purpose search engines discussed in Lesson 4 may be a good place to start gathering information. However, many of the major search engines index hundreds of millions of web pages, and the most carefully phrased search statement may sometimes produce an unmanageable number of results, or a list of results that are of poor quality. Another problem is that there are vast numbers of web resources, including the contents of searchable databases, that are completely "invisible" to the general purpose search engines. Actually, there are two distinct types of search tools that provide access to web resources: search engines and subject directories. As Lesson 4 explained, search engine indexes are created with automated programs and allow you to search for Web sites by keyword, using Boolean search parameters. Search engines usually have minimal human oversight and do not apply selection or evaluation criteria to the web pages they index. Subject directories (sometimes referred to as subject trees), are created by a person or persons who usually select sites using certain selection or evaluation criteria. Subject directories are hierarchically organized indexes of subjects and subheadings that allow the searcher to browse through lists of subjects for relevant information. Directories often are annotated with descriptions. Today, the line between search engines and subject directories has blurred. Search engines are partnering with subject directories, or creating their own directories, and returning results gathered from their spiders, as well as from a variety of other databases, guides and services. Some specialized search engines are actually searchable, hand-picked directories of web resources focusing on a particular subject. Many search engines and directory sites have expanded to become web portals: sites that offer a wide range of services and resources, such as web subject directories, white and yellow pages, online shopping malls, e-mail services, discussion forums, etc. Web portals emulate the services first offered by online service providers such as CompuServe and America Online. This lesson will help you to:
The "invisible web" consists of searchable databases, password-protected sites, and documents that are hidden by firewalls, which are inaccessible to the spiders and webcrawlers that compile indexes for the general purpose search engines. As web technology advances, web developers are creating more dynamic site interfaces and are providing more resources in searchable databases. When a search engine spider encounters a database, it can index only the location of the database, but nothing about the resources contained within it. These invisible resources are rapidly increasing, and many of their databases are maintained by educational institutions and government agencies, and contain a great deal of scholarly information. There are a number of subject directories providing access to the invisible web, including the following examples:
The invisible web directories listed above provide access to many specialized search engines. Specialized search tools may have interfaces that look identical to general search engines, but they function very differently, since there is usually a human interface that selects and sometimes annotates (or describes) the resources. Since they usually focus on a specific subject, a geographic region, or a certain type of computer file format, specialized search sites can dramatically reduce irrelevancy, and may help you to quickly pinpoint the information you need. Specialized search engines provide access to invisible web resources, but also index high-quality web sites available via general purpose search engines. These search engines usually index fewer Web pages, but focus only on information relevant to the topic. Another difference between large search engines and specialized search engines is human interaction. Many specialized search engines utilize subject specialists who evaluate and annotate each link, ensuring that only the most relevant and best quality resources are included. You can compare specialized search engines with specialized reference books in a library. You wouldn't go to a general encyclopedia to find an address or telephone number for an organization or association, but would need to consult a specialized encyclopedia of associations. Examples of specialized search engines include: Arts
& Humanities
Education
Health & Medical
Law & Legal
Science
Social Science
Software A number software
libraries provide searchable databases for freeware and shareware programs.
Most of the sites listed below also offer software reviews:
Subject directories are
usually compiled and maintained by people, or if by a computer program, by
some type of automated selection criteria. Like specialized search engines,
since they are usually maintained by human beings and are selective, subject
directory databases are smaller than those of the general purpose search
engines. Like the specialized search engines, directories usually produce
more relevant results than search engines because of their size and because
they usually index a web site's first page only. Although web subject
directories catalog a small segment of the Web's millions of documents, they
provide a quick and easy search by subject, and often by keyword. Directories
may be extremely useful if you have no idea where to start searching. They
are more useful for searching general subjects rather than for more specific
information. Beginning an information search in a subject directory can give
you some idea of the types of information files available on the Internet for
that particular subject. When beginning a search,
you will notice the top level subjects headings usually consist of very broad
subjects, such as "Arts and Humanities", "Education", and
"Health." After choosing a subject at the top level, you can move
through lists of submenus to narrow your search. Under "Health" you
might find "Diseases," "Drugs," and "Fitness."
Continue following the subheadings and eventually you will reach a page that
lists web documents. Click on the links that look interesting and use your
browser's Back button to return to the subject directory. Some of the subject directories
provide an alternative to moving down their hierarchical lists of menus by
providing a search engine for their database. You can use keywords to search
these directories, but you will be limited to resources in the directory's
database. Examples of general
subject guides include:
Remember that some of the
sites above also offer a web search engine in addition to their subject
directory of reviewed sites. The search engine may search a database which
includes non-reviewed sites compiled by an automated spider. There are several specialized
directories of subject guides compiled by subject specialists who are experts
in their subject fields. These directories are called distributed subject
trees. These directories distribute the responsibility of maintaining
lists of the best, most relevant Internet documents in various subject areas
to volunteers. Each volunteer is responsible for maintaining a list of
documents in his or her area of subject expertise. These guides are likely to
produce highly relevant information sources. Examples of distributed
subject trees include: There are also many
specialized directories that organize and provide links to resources in
specific subject areas, such as
File Format or Multimedia Searching One area that presents a
special problem for web researchers is locating resources in specific file
formats, including multimedia files such as shockwave, VRML, Java, pictures,
video files, or audio files, or particular file extensions such as .pdf. You
can search for these file formats by using one of the general purpose search
engines that provide multimedia searching, or you can use a specialized
search engine devoted to multimedia files or a particular type of file
format. These search engines can be useful when you need to locate multimedia
information, such as speeches, oral histories, recorded news events, or if
you want to find a music video or live radio broadcast. The Web is a rich source
for pictures and photographs, but be aware that most images on the web have
cryptic filenames that may not correspond to the subject of the image, such
as libimg.gif or comp.jpg, so a standard keyword search is not likely to be
successful. General searches usually do not produce audio or video files,
since the content of these files is not visible to the search engines. The following chart
provides a list of commonly found media file types found on the Web, along
with some of their extensions, and the player or software required to view or
listen. Some file formats are not supported by operating systems or web
browsers and require a browser plug-in.
The following general search
engines provide special functionality for searching various types of
multimedia formats: ·
AltaVista Multimedia Search allows you to
search for specific types of images, including photos, graphics, color or black
and white. Audio files are searchable audio files such as MP3, WAV, Windows
Media and Real Audio, and video files, including such formats as AVI, MPEG,
Quicktime, Windows Media, Real Video. You can even specify the length or
duration of audio and video files.
There are also
specialized, searchable directories devoted to multimedia resources:
Complete Exercise 5 after reading
Lesson 5. It is worth 7 points. Copyright © 1997-2000
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