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Lesson 3 Information retrieval services
ObjectiveDescribe the different categories of information retrieval services.

Information Retrieval Services

The information available online today is incredibly diverse, varying not only in subject matter, but also in format. Accordingly, the search services available reflect this diversity. Some are automated, and some rely on you to work your way through categories.
The first step, then, is to learn what kinds of information retrieval services are available to you.
Let us take a quick tour of representative sites in the main categories of information retrieval services before discussing how to search them:
  1. Tour Directories
  2. Tour Search Engines
  3. Tour Metasearch Engines
  4. Tour Subject pages and Link pages

The terms search engine and engine to refer to any service that allows you to compose your own search query. Any service that provides a compiled directory or allows you to perform searches is called a search service or information retrieval service.
As you now are aware, very often the question you will ask before beginning a search is not "Where do I find a search site?" but, rather, "Which one of all these services do I start with?" This is not a trivial question.
A directory, with its smaller number of hand-selected sites, may be more immediately useful than a search engine if you are searching for beginning-level information on a popular topic. A search engine, by its continual automated Web-roaming, may be more useful if you are looking for very specific information or an obscure topic.



Information Retrieval Models

Modeling in Information Retrieval is a complex process aimed at producing a ranking function
A ranking function is a function that assigns scores to documents with regard to a given query .
This process consists of two main tasks:
  1. The conception of a logical framework for representing documents and queries
  2. The definition of a ranking function that allows quantifying the similarities among documents and queries

Modeling and Ranking

Information Retrieval systems usually adopt index terms to index and retrieve documents
Index term:
  1. In a restricted sense: it is a keyword that has some meaning on its own; usually plays the role of a noun
  2. In a more general form: it is any word that appears in a document

  1. Retrieval based on index terms can be implemented efficiently
  2. Also, index terms are simple to refer to in a query
  3. Simplicity is important because it reduces the effort of query formulation

Retrieval Service Categories

Click the link below to review the categories of information retrieval services just viewed.
Retrieval Service Categories