Modern taxonomies that are used to support information management and findability are more similar to information retrieval thesauri and subject heading schemes than they are to classification systems. Another type of classification, the method of “faceted classification,” however, does apply to types of taxonomies. Facet means face, side, dimension, or aspect. In this sense, facets are meant to mean aspects of classification. A diamond, an object, or a digital content item is multi-faceted. A digital content item (text document, presentation, image, video, etc.) has multiple informational dimensions or aspects to it and thus multiple ways to be classified.
Classification is about putting an item, such as a content item (document, page, or digital asset) into a class or category. If it’s a physical object (a book) it goes onto a shelf of its class. In faceted classification, an item cannot physically be in more than one place, but it can still be “assigned to” more than one class. So, while the book itself can be on only one shelf, the record about the book can be assigned to more than one class.
Faceted classification assigns classes/categories/terms/concepts from each of multiple facets to a content item, allowing users to find the item by choosing the concepts from any one of the facets they consider first. Different users will consider different classification facets first. Users then narrow the search results by selecting concepts from additional facets in any order they wish, until they get a targeted result set meeting the criteria of multiple facet selections. The user interface of faceted classification is sometimes referred to as faceted browsing.
A standard feature of facets is that they are mutually exclusive. A concept/type belongs to only one facet. This is typical practice for the design of classification systems. The difference is that in faceted classification it is merely the concept/type/term that belongs to just one facet, not the content item or thing itself that would belong to only one classification in traditional classification systems.
The design, implementation, and use of facets to construct or refine searches have become so popular that it is no longer used just for classification aspects. Rather, a faceted taxonomy design may be used for any faceted grouping of concepts for search or metadata types that are relevant to the content and users. Faceted classification is intended to classify things that share all the same facets.
While faceted classification tends to be quite limited in the number of its facets, non-classification faceted taxonomies, whether based on subject types or separately controlled vocabularies, could result in a rather large number of facets. Faceted taxonomies that would not be considered faceted classification include those where multiple facets are created for organizing and breaking down subjects or when multiple facets are created for reflecting multiple different controlled vocabularies.
These faceted taxonomies stretch the meaning of “facet,” since the facets are not necessarily faces, dimensions, or aspects, but simply “types” suitable for filtering. They provide a form of guided navigation and are easier to browse and use than deep hierarchical taxonomies, so a large “subject” taxonomy could be broken down into specific subject-type facets.
While faceted taxonomies should also ideally be mutually exclusive, in contrast to the principle of faceted classification, the occasional exception of a concept belonging to more than one subject-type facet (question word of “What”) does not create a problem in the search. For example, the same concept Data catalogs, could be in the facet of Product Types and Technologies, as long as this type of polyhierarchy is kept to a minimum to avoid confusion. This would not be considered a case of classic polyhierarchy, because it’s not simply a matter of different broader concepts, but rather different facets or concept schemes. It is an attempt to address a different focus or approach to the topic that results in it being in more than one facet, offering an additional starting point for searchers.
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