Rhetorical Structure Theory

Rhetorical Structure Theory (RST) was originally formulated by William Mann and Sandra Thompson of the University of Southern California's Information Sciences Institute (ISI) in 1988. This theory was developed as part of studies of computer based text generation. Natural language researchers later began using RST in text summarization and other applications. RST addresses text organization by means of relations that hold between parts of text. It explains coherence by postulating a hierarchical, connected structure of texts.

In 2000, Daniel Marcu, also of ISI, demonstrated that practical discourse parsing and text summarization also could be achieved using RST. Marcu was named a fellow of the Association for Computational Linguistics in 2014 for his "significant contributions to discourse parsing, summarization, and machine translation and to kickstarting the statistical machine translation industry."

Rhetorical relations or coherence relations or discourse relations are paratactic (coordinate) or hypotactic (subordinate) relations that hold across two or more text spans. It is widely accepted that notion of coherence is through text relations like this. RST using rhetorical relations provide a systematic way for an analyst to analyse the text. An analysis is usually built by reading the text and constructing a tree using the relations. The following example is a title and summary, appearing at the top of an article in Scientific American magazine (Ramachandran and Anstis, 1986). The original text, broken into numbered units, is:

In the figure, numbers 1,2,3,4 show the corresponding units as explained above. The fourth unit and the third unit form a relation 'Means'. The fourth unit is the essential part of this relation, so it is called the nucleus of the relation and third unit is called the satellite of the relation. Similarly second unit to third and fourth unit is forming relation ′Condition'. All units are also spans and spans may be composed of more than one unit.

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