Automated Text Summarization and the SUMMARIST System

This paper consists of three parts: a preliminary typology of summaries in general; a description of the current and planned modules and performance of the SUMMARIST automated multilingual text summarization system being built sat ISI, and a discussion of three methods to evaluate summaries. 1. T H E N A T U R E O F S U M M A R I E S Early experimentation in the late 1950's and early 60's suggested that text summarization by computer was feasible though not s traightforward (Luhn, 59; Edmundson, 68). The methods developed then were fairly unsophisticated, relying primarily on surface level phenomena such as sentence position and word frequency counts, and focused on producing extracts (passages selected from the text, reproduced verbatim) rather than abstracts (interpreted portions of the text, newly generated). After a hiatus of some decades, the growing presence of large amounts of online text--in corpora and especially on the Web--renewed the interest in automated text summarization. During these intervening decades, progress in Natural Language Processing (NLP), coupled with great increases of computer memory and speed, made possible more sophisticated techniques, with very encouraging results. In the late 1990's, some relatively small research investments in the US (not more than 10 projects, including commercial efforts at Microsoft, Lexis-Nexis, Oracle, SRA, and TextWise, and university efforts at CMU, NMSU, UPenn, and USC/ISI) over three or four years have produced several systems that exhibit potential marketability, as well as several innovations that promise continued improvement. In addition, several recent workshops, a book collection, and several tutorials testify that automated text summarization has become a hot area. However, when one takes a moment to study the various systems and to consider what has really been achieved, one cannot help being struck by their underlying similarity, by the narrowness of their focus, and by the large numbers of unknown factors that surround the problem. For example, what precisely is a summary? No-one seems to know exactly. In our work, we use summary as the generic term and define it as follows: A summary is a text that is produced out of one or more (possibly multimedia) texts, that contains (some of) the same information of the original text(s), and that is no longer than half of the original text(s). To clarify the picture a little, we follow and extend (Sp~irck Jones, 97) by identifying the following aspects of variation. Any summary can be characterized by (at least) three major classes of characteristics: Invut: characteristics of the source text(s) Source size: single-document v s . multi-document: A single-document summary derives from a single input text (though the summarization process itself may employ information compiled earlier from other texts). A multi-document summary is one text that covers the content of more than one input text, and is usually used only when the input texts are thematically related. Specificity: domain-specific vs. general: When the input texts all pertain to a single domain, it may be appropr ia te to apply domain spec i f i c summarization techniques, focus on specific content, and output specific formats, compared to the general case. A domain-specific summary derives from input text(s) whose theme(s) pertain to a single restricted domain. As such, it can assume less term ambiguity, idiosyncratic word and grammar usage, specialized formatting, etc., and can reflect them in the summary.