Faceted taxonomies (taxonomies with attributes, dimensions, filters, etc. to limit search results based on the combination of selected criteria) are becoming increasingly popular with the support of web database technology. Unlike traditional hierarchical taxonomies, designing a faceted taxonomy first requires a decision on how many facets to create. There are various factors to take into consideration.
What the content supports
What the end-user user interface supports
Sometimes a website or intranet is created in a web content management system that does not give as much flexibility in taxonomy display. For example, SharePoint requires a horizontal list of facets, if the facets are to be used to filter content displayed in “columns,” where facet names are the column headers. Furthermore, SharePoint will by default create columns for document format type, content type, author, date created, and date modified. While you can hide these columns, if you want to use some of these defaults, that will limit the number of other descriptive facets for columns to about three or four.
Facets that limit search results are typically displayed in the left-margin, so more facets can be created. However, the number of facets should be limited so that all of the facet labels (although not necessarily all of their contents/facet values/terms) display by default without scrolling. The first 4-6 terms or values within a facet should be displayed to give the user a good understanding of what is in there, with a link or button to “show more.” Scrolling can be used when a facet category is expanded. So, what needs to be considered is the vertical space if all facets display at least some values, and if that does not fit, whether some facets can be collapsed by default. The example below of the facets for limiting people search results on LinkedIn shows the default display of two facets with the first 6 terms, one facet with all 5 terms, and 12 facets collapsed (an unusually high number of facets).
What the tagging process supports
Organizations which tag/index content for subscription sale, on the other hand, where content indexing is core to their business, will invest in dedicated indexers who can be given thorough training in assigning terms from multiple facets and will also check their indexing for quality. Thus, for professional indexing, a greater number of facets can be supported.
In automated tagging, it’s not so much a matter of how many facets, but rather how distinct the facets are and how easy they are for automated tagging. There are different technologies out there, but, in general, named entities/proper nouns are easier to distinguish than topical subjects. So, facets for author, location, department, product name, etc., are easy to classify automatically. Language, and a document type that is based on file format are also straight-forward for auto-classification. Subject or Topic could be catch-all for high-ranked keywords. If you want to create facets for different kinds of topics, though, such as Purpose, Activity, Significance, Origin, etc., the distinctions will likely be too challenging for an auto-classification tool.