Today’s question comes from Winona, and references a previous reflection I wrote. Winona asks:

In your post on the subjective side of indexing, you write that two indexers can produce different indexes for the same book, but that there should still be significant overlap because the text’s aboutness remains the same. Is there any practical way to quantify that overlap? For instance, after accounting for defensible differences in wording, formatting, and structure, should a certain percentage — say, 50% or more — of top-level entries overlap?

Thank you, Winona. That is an excellent question. If a certain amount of overlap is expected, it is reasonable to ask if that overlap can be quantified.

My short answer is no, I do not know if there is a practical way to quantify that overlap. I am also not sure if it is possible to ever fully quantify it. I imagine a lot would depend on the subject matter and type of book, as well as, of course, the subjective approach each indexer brings.

To attempt a quantitative answer, I think you would need to analyze multiple indexes from multiple indexers across several books. I have not done that research. If someone else would like to do so, I think the results would be fascinating.

I will, however, still attempt a response. While I do not have hard numbers, this is a question worth considering and I have some thoughts about what to look for in terms of overlap. I am mostly drawing upon my experience working with subcontractors. While subcontracting is not an exact comparison, since there is usually one indexer providing direction, subcontracting does involve two or more indexers working on the same text.

I also highly recommend this article by Jolanta Komornicka, recently published in The Indexer. Jolanta convened two panels, at two different conferences, for which participating indexers indexed the same book. Jolanta attempts to answer this very question, of how similar or different the resulting indexes are. Definitely worth checking out.

When considering overlap between indexes for the same book, I want to distinguish between main headings and everything else.

At the main heading level, I would expect a high level of overlap. Most terms are not up for interpretation. People, places, organizations, and objects should generally be indexed as written in the text. Concepts which are clearly identified in the text should also usually be indexed as presented.

There is still some room for variation and the indexers’ subjective choices. For example, whether or not to include glosses, and how to phrase the glosses; how to handle synonymous terms; what counts as a passing mention, and, conversely, which are important themes and discussions; and any discussions which are implicit and which require more interpretation. But by and large, I would expect main headings to mostly be similar across all indexes. 

Beyond main headings, I think there is far more room for different decisions to be made. These include:

• Metatopic: How is the metatopic handled?
• Structure: How is information divided or gathered throughout the index?
• Subheadings: How many subheadings are used? Are subheadings mixed with undifferentiated locators? How are subheadings phrased?
• Cross-references: Are cross-references used sparingly or extensively? In which directions are cross-references pointing? Are cross-references used alongside or instead of double-posts?

At the level of structure and subheadings, there are so many different decisions that can be made. There should be recognizable overlap in terms of themes and correspondence to the text, but I don’t know if it is possible to quantify how much overlap or to expect that certain subheadings and arrays will be interchangeable. 

To sum up, I would expect a high level of overlap between main headings, since many terms should be indexed as is. But indexing is a lot more than simply identifying and picking up terms. An excellent index involves organizing all of those terms in a way that is accessible and to highlight the key themes of the book. How all that organization happens is very much subjective. While the indexes should all still clearly point back to the same text, I don’t know if it is possible to fully quantify overlap.

Shifting directions slightly: I think what the creators of AI indexing tools are attempting to do is to quantify or codify indexing decisions. Or at least they believe that a good index is simply the result of following a certain set of rules. But that is not entirely true. Yes, an index is deeply governed by rules and conventions, and an AI tool can probably reproduce those rules and mimic various techniques. But an excellent index is ultimately the result of the indexer’s judgment. It is deciding which rules to apply, and when, and how to combine techniques, and how to make adjustments to fit the material rather than trying to force the material to fit the format. The subjective nature of writing an index, while difficult to quantify and difficult to compare, is also a superpower, enabling indexes and indexers to respond to the text and to readers. 

PS. As I mentioned, I highly recommend Jolanta Komornicka’s recent article in which Jolanta attempts to quantify how similar and different the indexes by her panelists are. You can find it here. If you do not have a subscription to The Indexer, try accessing through your local library. 

I’ve been thinking about the saying that two indexers can index the same book and write two very different indexes. Which is true. This was demonstrated during the panel moderated last year by Jolanta Komornicka, at both the Indexing Society of Canada/Société canadienne d’indexation (ISC/SCI) and Society of Indexers (SI) conferences. For the panel, a number of experienced indexers indexed the book The Claremont Run: Subverting Gender in the X-Men, by J. Andrew Deman (University of Texas Press, 2023), and compared and discussed their decision-making, including how their indexes differed. I was not part of the panel, though I very much enjoyed the discussion at the ISC/SCI conference in Vancouver.

For all the differences—and each indexer did have their own approach—the overlap was also striking. It reminded me of something Enid Zafran said, that even though two indexers will write different indexes, there should still be significant overlap because the text itself remains the same. Everything that is in the book still needs to somehow be reflected in the index.

I am going to call what’s in the book aboutness, to use index-speak. In my book, Book Indexing: A Step-by-Step Guide, I define aboutness as “understanding what the text, at all levels, is about.” Meaning, what is the book as a whole about? What are the main concepts? Who are the relevant people? What organizations are discussed, or court cases cited, etc…? All of these discussions and mentions need to be in the index. How they appear in the index may differ, but one way or another, the contents of the book should be recognizable and findable. 

So there is this tension, in writing an index, between the objective and subjective elements. The text is the text. The rules and conventions are the rules and conventions. But the application and interpretation will vary.

I’m conscious of this tension when I subcontract. I want certain elements handled a certain way, while also aware that the subcontractor may have a different view or may be dealing with a different preference if they also subcontract for another indexer.

I think this tension also exists for anyone who has served on a jury for an indexing award. When I was on the Award Committee for the Ewart-Daveluy Award, some submissions were written differently than how I would have written them. Yet the award is not for, “who can best mimic how the judges would have written the index.” The award is for excellence, firstly for how well the index reflects the book, and secondly for how well the elements and strategies used in the index work. As a judge, I need to put aside my personal approach and judge submissions on their own merits, accepting that best practices can be interpreted and implemented in a variety of ways.

So what does all this mean for you or I as indexers?

First of all, the objective side of indexing is important. It is important to understand the book and accurately point back to it. The formatting and other conventions exist for a reason. Rules can be broken, but any deviation should be well thought out and justified.

It is also okay, however, to develop your own style. I know it sounds weird to discuss indexing in terms of style, but I think our personal preferences and methods do add up to make our indexes unique. If we are going to push back against a bland average that AI seems to excel at, maybe we need to talk more about what makes our human-written indexes stand out.

So as you practice and write indexes, consider. Of various approaches available, which do you like the best and why? What works better in certain scenarios and not others? What looks more elegant? What saves space? What do you want to borrow from others, and what do you disagree with? While keeping an eye on the goal of creating excellent indexes for readers, put your own stamp on your indexes too.