Duplicate entries versus see cross-references in back-of-book indexes

When there is a choice, does a back-of-book indexer use a duplicate entry or a see reference? Guidelines suggest that it is preferable to use the duplicate entry if it would not add to the length or complexity of the index. The author studied 1,100 see references in 202 back-of-book indexes and concluded that 22% of the see references should have been replaced by duplicate entries. Failure to select a duplicate entry instead of a see reference occurs most frequently in science and technology books and in indexes with no subheadings.