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Search units

In information retrieval, we distinguish three kinds of search units:

  1. Individual documents: the focus of the search is on a specific document and not on a list of possible documents.
  2. Any document: the focus is on any document containing a specific character string defined in a search inquiry.
  3. Information on a specific subject: the focus is on documents detailing the given subject (concept).

For each kind of search we can identify typical user applications. In case 1, the user knows that there is one single document which contains the key information he is looking for. In case 2, the user wants to study (analyse) the contexts of character strings, but not the information which may be communicated by the retrieved documents; typical users are terminologists and linguists. In case 3, the user does not want a unique document: the information he wants must belong to the selected subject about which he hopes to improve his knowledge.

Thus, the application areas for IR are very diverse. We may distinguish between the following search methods, based on the kind of search:

Reference text search (bibliographic IR):
Besides standard data and fact retrieval, this would include specialised applications, such as IR for specific storage media (e.g. compact disks) or mixed media (e.g. image retrieval), and IR combined with problem solving (e.g. expert systems).
Non-reference text search (full text systems):
This would include IR for on-line product information, reports, journals, etc., and for short lived information (e.g. electronic mail) that could be stored in personal databases or in a company's archives.
Specialised text search (texts generated for electronic use and support):
This would include IR for encyclopaedic information (short technical texts) and for hypertext (non-linear structured text).