1. Computer-readable code which is operable, when used to control an electronic computer, to identify descriptive words contained in a digitally encoded input text, by the steps of (i) processing the input text to generate a list of text words, (ii) selecting a text word from (i) as a descriptive word if that word has an above-threshold selectivity value in at least one library of texts in a field, where the selectivity value of a word in a library of texts in a field is related to the frequency of occurrence of that word in said library, relative to the frequency of occurrence of the same word in one or more other libraries of texts in one or more other fields, respectively, and (iii) storing or displaying the words selected in (ii) as descriptive words.