While one purpose of an index is to bring information together, the indexer must take care not to become too obsessed with gathering every bit of related information into larger chunks, a practice known as classification. For example, in a classified index subentries for cattle would be listed under bulls and cows. Instead of classifying subheadings, an indexer could make main headings for each of the subheadings. A See reference could be included from cattle to bulls and cows.
Classified entries work well for the reader as long as the subject being subdivided is not too general and as long as the entry does not continue for column after column. Readers appreciate having all relevant information gathered in one place. Readers may find it is just as easy to scan a list of classified subheadings as it is to scan a long list of unclassified subheadings. It is preferable to make classified entries rather than leaving readers guessing whether they have found all the relevant information that might otherwise be scattered throughout the index.
For more information about classification in indexes, see Linda K. Fetters’ Handbook of Indexing Techniques, Fifth Edition. Information Today, Inc. Medford, New Jersey, 2013, pp.30-32.
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