Page 153 - DLIS408_INFORMATION_TECHNOLOGY-APPLICATIONSL SCIENCES

Information Technology and Application

Notes These pages are retrieved by a Web crawler (sometimes also known as a spider) — an automated
Web browser who follows every link on the site. Exclusions can be made by the use of robots.txt.
The contents of each page are then analyzed to determine how it should be indexed (for example,
words are extracted from the titles, headings, or special fields called meta tags). Data about web
pages are stored in an index database for use in later queries. A query can be a single word. The
purpose of an index is to allow information to be found as quickly as possible. Some search
engines, such as Google, store all or part of the source page (referred to as a cache) as well as
information about the web pages, whereas others, such as AltaVista, store every word of every
page they find. This cached page always holds the actual search text since it is the one that was
actually indexed, so it can be very useful when the content of the current page has been updated
and the search terms are no longer in it. This problem might be considered to be a mild form of
linkrot, and Google’s handling of it increases usability by satisfying user expectations that the
search terms will be on the returned webpage. This satisfies the principle of least astonishment
since the user normally expects the search terms to be on the returned pages. Increased search
relevance makes these cached pages very useful, even beyond the fact that they may contain data
that may no longer be available elsewhere.
When a user enters a query into a search engine (typically by using key words), the engine
examines its index and provides a listing of best-matching web pages according to its criteria,
usually with a short summary containing the document’s title and sometimes parts of the text. The
index is built from the information stored with the data and the method by which the information
is indexed. Unfortunately, there are currently no known public search engines that allow documents
to be searched by date. Most search engines support the use of the boolean operators AND, OR and
NOT to further specify the search query. Boolean operators are for literal searches that allow the
user to refine and extend the terms of the search. The engine looks for the words or phrases exactly
as entered. Some search engines provide an advanced feature called proximity search which
allows users to define the distance between keywords. There is also concept-based searching
where the research involves using statistical analysis on pages containing the words or phrases
you search for. As well, natural language queries allow the user to type a question in the same
form one would ask it to a human. A site like this would be ask.com.

Task Discuss how web search engine works.
The usefulness of a search engine depends on the relevance of the result set it gives back. While
there may be millions of web pages that include a particular word or phrase, some pages may be
more relevant, popular, or authoritative than others. Most search engines employ methods to
rank the results to provide the “best” results first. How a search engine decides which pages are the
best matches, and what order the results should be shown in, varies widely from one engine to
another. The methods also change over time as Internet usage changes and new techniques evolve.
There are two main types of search engine that have evolved: one is a system of predefined and
hierarchically ordered keywords that humans have programmed extensively. The other is a system
that generates an “inverted index” by analyzing texts it locates. This second form relies much
more heavily on the computer itself to do the bulk of the work.
Most Web search engines are commercial ventures supported by advertising revenue and, as a
result, some employ the practice of allowing advertisers to pay money to have their listings
ranked higher in search results. Those search engines which do not accept money for their search
engine results make money by running search related ads alongside the regular search engine
results. The search engines make money everytime if someone clicks on one of these ads.

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