Editors and publishers want to know: What makes a good index?

In an interview with publisher Sandra Uschtrin, indexer Jochen Fassbender described some key qualities of a good index.

What does a good index look like?

The key qualities include:

Good term selection: The heart of quality indexing. Entries must be clear, meaningful, accurate to the text, and complete with locators. This is why indexing of a print book should take place only after the text pagination is final.
Comprehensive and consistent coverage of all important details and passages.
Entries for both implicit and explicit subjects (i.e., not simply a list of names and keywords).
All references to a topic consolidated under one “preferred” term, even if the text uses multiple terms to refer to that topic
Synonyms added to increase access points, either pointing to the preferred term via “see” cross-references or repeating the preferred term’s locators (double-posting).
Related index entries point to each other with “see also” cross-references.
Absence of passing mentions: This means the indexer did not index a word that is only in the text for illustrative purposes or otherwise does not provide significant information.
Accurate and explicit locators: Commas and dashes are used to distinguish between intermittent discussions of a topic on consecutive pages (e.g., 514, 515, 516) and continuous discussion of a topic on consecutive pages (e.g., 635-637). Furthermore, the start and end of page ranges are listed explicitly (e.g., 635-637 instead of 635 ff).
Ideally, main entries with more than 5 or 6 locators broken down into subheadings.
Appropriate length: Typically 4-5% of the length of the indexable material, and even 10% or more for reference works. Below 3.5% may be problematic.

Indexes with these traits add value to the text by ensuring users can efficiently find all substantive concepts, details, and facts.

What should editors and publishers look for in an indexer?

Read the complete interview—including Fassbender’s take on the “deadly sins” he wants indexers to avoid—for free in The Indexer: The International Journal of Indexing (Vol. 29, No. 1) liverpooluniversitypress.co.uk/doi/10.3828/indexer.2011.4

Fassbender also talked about some of the things that an indexer needs to produce quality work:

Training and practice: Essential for producing good analytical indexes
Ability to anticipate users’ needs and questions
Some knowledge in the book’s subject area
Indexing software: Indispensable for increasing the speed of indexing by handling and automating technical components