In-depth indexing is based on anticipating the needs of a reader. At the very least, an index needs enough primary entries to cover the primary topics of the work. An in-depth index will also contain enough secondary entries to cover commonly used synonyms and related words. Primary entries that cover a broad topic or a wide page range should have enough subentries to ensure that they refer to a manageable amount of information for the reader.
One of the most important features of an in-depth index is the use of synonyms and related words to create multiple access points. For example, a shallow index may have "computers" as a primary reference, with no secondary references pointing to it. A reader looking through the index for entries on PCs, laptops or electronic storage devices may get the erroneous impression that the book contains no relevant information. A deep index will include secondary entries such as "PC; see Computer" which can help put a researcher on the right track without having to outguess the book's authors.
A primary reference entry with a long string of page numbers can present a daunting research task, particularly if it is a broad topic. By sorting the page numbers into more specific sub-entries, a reader can more easily tell which pages are likely to contain useful information. For example, a primary entry of a broad topic such as "Food" followed by a dozen page numbers is unhelpful. If those page numbers can be separated into sub-entries such as "history of," "recipes," "restaurants" or even "avocados," a researcher can more quickly determine which pages they are interested in.
If a book primarily deals with a specific topic such as the Great Chicago Fire, it may be tempting to leave the small section dealing with small-scale dairy operations out of the index entirely. However, a researcher who is gathering information on small-scale dairy operations is precisely the sort of reader who will most benefit from an index entry on the subject, since they are not interested in the bulk of the material and are unlikely to read the book all the way through. An in-depth index that includes this material will be of much greater use to this kind of researcher.