Accurate methods for the statistics of surprise and coincidence
Computational Linguistics - Special issue on using large corpora: I
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WordSmith Tools (Scott, 1998) offers a program for comparing corpora, known as KeyWords. KeyWords compares a word list extracted from what has been called 'the study corpus' (the corpus which the researcher is interested in describing) with a word list made from a reference corpus. The only requirement for a word list to be accepted as reference corpus by the software is that must be larger than the study corpus, one of the most pressing questions with respect to using KeyWords seems to be what would be the ideal size of a reference corpus. The aim of this paper is thus to propose answers to this question. Five English corpora were compared to reference corpora of various sizes (varying from two to 100 times larger than the study corpus). The results indicate that a reference corpus that is five times as large as the study corpus yielded a larger number of keywords than a smaller reference corpus. Corpora larger than five times the size of the study corpus yielded similar amounts of keywords. The implication is that a larger reference corpus is not always better than a smaller one, for WordSmith Tools Keywords analysis, while a reference corpus that is less than five times the size of the study corpus may not be reliable. There seems to be no need for using extremely large reference corpora, given that the number of keywords yielded do not seem to change by using corpora larger than five times the size of the study corpus.