Bottom up and top down are two opposite (but completely compatible) approaches to developing hierarchical structure. The controlled vocabulary standard ANSI/NISO Z39.19 explains the two approaches as follows:

a) Top Down – The broadest terms are identified first and then narrower terms are selected to reach the desired level of specificity. The necessary hierarchical structures and relationships are created as the work proceeds.

b) Bottom Up – This case frequently occurs when lists of terms have been derived from a corpus of content objects and are then to be incorporated in a controlled vocabulary. As in the case above, the necessary hierarchical structures and relationships are created as the work proceeds, but starting from the terms having the narrowest scope and moving to the more generic ones.

The standard adds, “If a new controlled vocabulary is being created, the “top down” approach is preferred. Once a controlled vocabulary is in place, the “bottom up” approach is most often used to add new terms to cover new concepts.” (ANSI/NISO Z39.19-2005, page 91)

Eminent taxonomist Jessica Milstead explains the process of thesaurus construction largely in terms of following either “The Top-Down Method” or “The Bottom-Up Method”, in her short article on the American Society for Indexing website, “How Do I Build a Thesaurus?”

Well-known information management consultant Leonard Will has a somewhat different view of how to use the two approaches:

I don’t think that there is only one way “logical” way of building a hierarchical structure of concepts. You can do it “top down”, by starting with broad categories and populating them with terms to represent narrower and narrower concepts.

You can also do it “bottom up”, but collecting terms (from previous schemes and user suggestions, from the literature, from queries and so on), sorting these concepts into facets and then grouping these into hierarchies.

In practice I generally use a combination of both these approaches.

(Leonard Will, correspondence in the JISCMail – TAXONOMY Archives)

In my own experience in creating and developing taxonomies, I have also found that a combination of top down and bottom up approaches works best. It also seems to be the most natural and organic approach.

Marjorie M.K. Hlava President, Access Innovations

This posting is one of a series based on a workshop, “Thesaurus Creation and Management,” that Marjorie Hlava presented in December of 2012.