Based on accessibility of information, two broad categories of information retrieval systems can be identified: in-house and online. In-house information retrieval systems are set up by a particular library or information centre to serve mainly the users within the organization. One particular type of in-house database is the library catalogue. Online public access catalogues (OPACs) provide facilities for library users to carry out online catalogue searches, and then to check the availability of the item required.
By online information retrieval systems we mean those that have been designed to provide access to remote database(s) to a variety of users. Such services are available mostly on a commercial basis, and there are a number of vendors that handle this sort of service. With the development of optical storage technology, another type of information retrieval system appeared on CD-ROM (compactdisc read-only memory). Information retrieval systems based on CD-ROM technology are available mostly on a commercial basis, though there have been some free and in-house developments too. Basic techniques for search and retrieval of information from the in-house or CD-ROM and online information retrieval systems are more or less the same, except that the online system is linked to users at a distance through the electronic communication network.
Recent developments in computer and communication technologies have widened the scope of online information retrieval systems. The Internet and World Wide Web have made information available for use by anyone virtually anywhere with access to the appropriate equipment. This has led to the concept of a digital global library system where information can be generated and made available in electronic form on the Web for use by any user from any corner of the world. This of course involves a number of technical and management issues that need to be resolved in order to make the global digital library concept a reality.
In today’s digital library world, a user can get access to different types of information sources in a variety of formats. For example, a digital library may contain simple catalogues of information resources, like OPACs (Online Public Access Catalogues), or may contain full texts of documents, images, audio and video materials. The information resources may be available in different formats, and they may have been produced by using different types of hardware and software.
For example, the text may be in MS-Word, or PDF, or in HTML format; images may be available in GIF or JPEG file formats. These information resources may reside on a number of different servers – local as well as remote – and they may have been indexed differently. All these issues make the information retrieval process very complex.
Traditional online information search systems that began about four decades ago were designed to provide access to remote databases, often through a database vendor or service provider. These systems were expensive to use. They were not quite suitable for searching directly by the end-users, and in most cases were used by information intermediaries on behalf of, or in cooperation with, the endusers. Online search services have been provided by database producers, but more commonly by service providers or vendors like Dialog, Ovid, etc. The major characteristics of this type of online information retrieval system are as follows:
❖ users get access to remote databases that are often many in number and large in size;
❖ many databases can be searched using a single search interface;
❖ database records mainly contain bibliographic details of records with abstracts, and sometimes with additional information, such as citations, etc.; only some databases contain full text information;
❖ service providers have their own search interface with good search and retrieval capabilities;
❖ users need to register with the service providers;
❖ users are charged for searching as well as for the content; and
❖ modern online service providers have web interfaces with good search features and hyperlinked records/information.
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