Document retrieval is defined as the matching of some stated user query against a set of

free-text In Document retrieval, text retrieval, full-text search refers to techniques for searching a single computer-stored document or a collection in a full-text database. Full-text search is distinguished from searches based on metadata or on parts of ...

records. These records could be any type of mainly unstructured text, such as newspaper articles, real estate records or paragraphs in a manual. User queries can range from multi-sentence full descriptions of an information need to a few words. Document retrieval is sometimes referred to as, or as a branch of, text retrieval. Text retrieval is a branch of information retrieval where the information is stored primarily in the form of

text Text may refer to: Written word * Text (literary theory), any object that can be read, including: **Religious text, a writing that a religious tradition considers to be sacred **Text, a verse or passage from scripture used in expository preachin ...

. Text databases became decentralized thanks to the

personal computer A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or tec ...

. Text retrieval is a critical area of study today, since it is the fundamental basis of all

internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...

search engines.

Description

Document retrieval systems find information to given criteria by matching text records (''documents'') against user queries, as opposed to expert systems that answer questions by inferring over a logical knowledge database. A document retrieval system consists of a database of documents, a classification algorithm to build a full text index, and a user interface to access the database. A document retrieval system has two main tasks: # Find relevant documents to user queries # Evaluate the matching results and sort them according to relevance, using algorithms such as

PageRank PageRank (PR) is an algorithm used by Google Search to rank webpages, web pages in their search engine results. It is named after both the term "web page" and co-founder Larry Page. PageRank is a way of measuring the importance of website pages. A ...

. Internet

search engines A search engine is a software system designed to carry out web searches. They search the World Wide Web in a systematic way for particular information specified in a textual web search query. The search results are generally presented in a ...

are classical applications of document retrieval. The vast majority of retrieval systems currently in use range from simple Boolean systems through to systems using statistical or natural language processing techniques.

Variations

There are two main classes of indexing schemata for document retrieval systems: ''form based'' (or ''word based''), and ''content based'' indexing. The document classification scheme (or indexing algorithm) in use determines the nature of the document retrieval system.

Form based

Form based document retrieval addresses the exact syntactic properties of a text, comparable to substring matching in string searches. The text is generally unstructured and not necessarily in a natural language, the system could for example be used to process large sets of chemical representations in molecular biology. A suffix tree algorithm is an example for form based indexing.

Content based

The content based approach exploits semantic connections between documents and parts thereof, and semantic connections between queries and documents. Most content based document retrieval systems use an

inverted index In computer science, an inverted index (also referred to as a postings list, postings file, or inverted file) is a database index storing a mapping from content, such as words or numbers, to its locations in a table, or in a document or a set of d ...

algorithm. A ''signature file'' is a technique that creates a ''quick and dirty'' filter, for example a Bloom filter, that will keep all the documents that match to the query and ''hopefully'' a few ones that do not. The way this is done is by creating for each file a signature, typically a hash coded version. One method is superimposed coding. A post-processing step is done to discard the false alarms. Since in most cases this structure is inferior to

inverted file In computer science, an inverted index (also referred to as a postings list, postings file, or inverted file) is a database index storing a mapping from content, such as words or numbers, to its locations in a table, or in a document or a set of do ...

s in terms of speed, size and functionality, it is not used widely. However, with proper parameters it can beat the inverted files in certain environments.

Example: PubMed

The

PubMed PubMed is a free search engine accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics. The United States National Library of Medicine (NLM) at the National Institutes of Health maintain t ...

form interface features the "related articles" search which works through a comparison of words from the documents' title, abstract, and

MeSH A mesh is a barrier made of connected strands of metal, fiber, or other flexible or ductile materials. A mesh is similar to a web or a net in that it has many attached or woven strands. Types * A plastic mesh may be extruded, oriented, exp ...

terms using a word-weighted algorithm.

References

External links

{{Commons category
Formal Foundation of Information Retrieval
Buckinghamshire Chilterns University College Information retrieval genres Electronic documents Substring indices Search engine software