Specificity is one of the elements we need to strive for in an index. We are, after all, as indexers, in the business of directing people to information, and to do so accurately and efficiently entries needs to be specific enough for the reader to quickly grasp. But this can still raise the question of, how much specificity is necessary?

We can think of information as being on a spectrum from most general to most specific. The trick for indexing is to know which points on the spectrum should make it into the index. Put another way, we can ask, should the object be named or described? When discussing cars, for example, is a more general entry for automobile industry more appropriate, or should there be entries for specific brands or models? In a way, both options are specific in their own ways, and there are a few factors to consider when writing entries.

The simplest answer is to include all options in the index, both general and specific. This provides multiple entry points, which is good indexing practice. This should allow most, if not all, users to find what they want, regardless of the search terms they use. This can also be a good exercise for thinking of synonyms to include as cross-references. But a particular audience may not need or want all options, nor may there be enough space. So there are some other considerations.

Audience, as mentioned, is of course another factor to consider. If the book is written for a niche audience, then that audience will probably know the jargon and other specific terms, which suggests that more general terms can be left out. Similarly, a book used for reference would probably benefit from a detailed, specific index, because readers will already know what they want before they read the book. Conversely, a trade book may require both specific and descriptive entries, as there may be readers not familiar with the specific names of things. 

Context is also important, both in the text and in the index. Is the point being made about the automobile industry in general, with selected companies for examples, or is the focus on the specific companies? We can also consider how much space is given to the discussion in the text, with a larger discussion perhaps meriting more entry points. 

In the index itself, we can also play around with a range of specificity among different headings and subheadings. One strategy I use is to have a more general subheading to capture a range of related material while also providing specific main entries. I think this can be a good middle ground as it provides multiple entry points while keeping the index a reasonable length through a few descriptive, general entries. For example, entries on presidential views on foreign affairs could read:

• Bush, George W.: on foreign affairs
• Clinton, Bill: on foreign affairs
• foreign affairs: presidents on

The space for the index can also, of course, be a consideration. Being very specific can lead to a longer index, so in some cases a more general entry with a broader reach may be preferable. This approach may not be ideal, as it loses alternative entry points, but if space is an issue I prefer to cast a wider net than focus on a subset of content and ignore the rest. 

As a general rule, aim to be specific. Readers should not be left guessing. That said, specificity exists on a spectrum, and where you land depends on the audience, the text, and how much space you have for the index. Find where these factors intersect and you will find your sweet spot.