Ratio of Relative Frequencies was suggested by Damerau (1993). It considers only the relative frequency of features and relies on the division of the value for the target corpus by the value of the comparison corpus. It can’t deal with words that don’t appear in the comparison corpus.
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Gries, Stefan Th., ‘Useful Statistics for Corpus Linguistics’, in A Mosaic of Corpus Linguistics: Selected Approaches, ed. by Aquilino Sánchez and Moisés Almela (Frankfurt am Main: Peter Lang, 2010), pp. 269–91
Damerau, Fred J., ‘Generating and Evaluating Domain-Oriented Multi-Word Terms from Texts’, Information Processing & Management, 29.4 (1993), 433–47