Sunday, February 25, 2018

Taxonomies for Filtering and Sorting

Taxonomies are versatile and may be used for various purposes. Originally designed to support hierarchical browsing of topics linked to content, they also may be implemented to support more accuracy in searching. Most discussions of taxonomies have focused on browse and/or search, but taxonomies may function in additional ways: enhancing filtering and sorting.


Taxonomies structured into facets serve a combination of search and browse, and thus serve what is often called “faceted search” or “faceted browse” (as described in a previous blog post, Faceted Search vs. Faceted Browse). However, it’s simpler, more accurate, and more helpful in understanding facets to consider a faceted taxonomy as serving a distinct role from browsing or searching, that of filtering.

Filtering is a common function which is not limited to use with taxonomies. Filtering can be done by non-taxonomy attributes, such as keyword, author, date, etc., if these are set up as metadata and implemented as filters. We see filters in situations which lack taxonomies in options in the email inbox or lists of documents in file management user interfaces. We also find filters on documents in SharePoint libraries and other content/document management systems, which may include taxonomies. In a user interface, the icon for filtering is a funnel (where you can imagine that it is lined with a cone-shaped filter paper).

Filters may be known by other names, such as “Refinements”/“Refine by” or “Limit by.” These designations may be used interchangeably, although they tend to be used different circumstances. “Filtering” may be done on search results or on a complete set of records, such as a list in a spreadsheet or table. “Refining” or “limiting,” on the other hand, would usually be performed only on the results of an executed search, as a further refinement or limiting factor on an initial set of search results which turned out too large. “Refining,” furthermore, suggests a more careful search process, so this name is more often used in research databases or other repositories of articles and resources.

A relatively small faceted taxonomy comprising short lists of terms for each facet/filter, from which the user can select from a displayed list or drop-down, is both easy to use and, with proper tagging, can achieve accurate retrieval results.


Sorting is done on content in a spreadsheet or table, where data on content items is in different columns, with sorting done by an attribute of an individually selected column. Sorting could be by numeric order, by alphabetic order, by date, or by the mere presence of a binary value.  Indeed, most sorting does not involve taxonomies, but it can.  If a column is for “Topic” and items have been tagged with taxonomy terms, then the items can be sorted by taxonomy term topic.

The function of sorting with taxonomy terms may not be quite as common as filtering, since it is not done on search results but only on data in a table or spreadsheet. However, in many situations content items are presented this way: content in spreadsheets and databases, messages in an email inbox, content items in SharePoint libraries and various kinds of content management systems, and many other applications. Furthermore, it’s often simpler to sort than to filter.

Sorting is nevertheless a function associated with taxonomies, at least in the definition of taxonomy in the ISO standard for controlled vocabularies. ISO 25964-2:2013 in section 3.83 defines taxonomy as a “scheme of categories (3.5) and subcategories that can be used to sort and otherwise organize items of knowledge or information.”

The following screenshot from SharePoint shows the availability of both sorting (by clicking on the down-arrow next to a column name, such as Topic), and filtering (by selecting Topics in the filter pane on the right).


As taxonomies are versatile, the same taxonomy can be used for multiple purposes: browsing, searching, filtering, and sorting. However, a taxonomy design usually can be optimized for the user experience of only one or two implementations. So, a taxonomy that delivers a great user experience for hierarchical browsing, might not be best suited for filtering or sorting, or vice versa. Filtering is more accommodating for various taxonomies than is sorting, since it may not be necessary to display the entire taxonomy in filters.

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