In my experience, people are better at building hierarchies than they are at using hierarchies. They’re an effective way for the builder to form new knowledge and interrogate what they already know, but very hard for others to understand. Microsoft Learn has hundreds of hierarchies in the form of tables of contents all over the site. These are very helpful to authors and content managers, where they act as maps of content. They’re much less useful to users.
Hierarchies are easy to create but hard to understand. This is why it’s so essential to keep hierarchies we present to others flat, not deep. It’s not really that flat hierarchies are better, it’s that they’re less hierarchy, which is usually easier to work with.
If you have clear items arranged into ambiguous categories, you want to have shallow, broad structures. When we say ambiguous categories, we mean categories that could be interpreted in multiple ways (which I believe is the case for almost all TOC groups.) An example would be “Western States > Colorado.” Is Colorado western? Is it Mountain? It’s kind of in the middle of the country. It’s not a bad category, necessarily, it’s just a little ambiguous.
If you have unambiguous categories, deeper, narrower hierarchies work better. For instance, in the hierarchy “United States > Colorado > Denver,” each of those things is in an obvious place and could not be in another.
If none of the labels (categories or document titles) are likely to be clear to users, a deeper structure might be better. If this is what you’re working with, though, you need more than a hierarchy, because everybody is just guessing at every stage.
Golledge, Reginald G.https://doi.org/10.1111/1467-8306.00276.Annals of the Association of American Geographers, vol. 92, no. 1, 2002, pp. 1–14. Wiley Online Library,
Hierarchies are easy to create but hard to understand. We cognitively sort things into hierarchies and work overtime to meaningfully cluster and categorize the spatial information we get. p 8
Miller, Craig S., and Roger W. Remington. “Modeling Information Navigation: Implications for Information Architecture.” Human–Computer Interaction, vol. 19, no. 3, Sept. 2004, pp. 225–71. DOI.org (Crossref), https://doi.org/10.1207/s15327051hci1903_2.
Whitenton, Kathryn. “Flat vs. Deep Website Hierarchies.” Nielsen Norman Group, https://www.nngroup.com/articles/flat-vs-deep-hierarchy/. Accessed 8 June 2023.