Hierarchical taxonomies are best suited to things where the relationship is well known. Denton (2009) wrote an article about faceted classification that talks about when to not use a faceted classification. He says,
Hierarchies and trees (imagine indented lists) are best when the entities in question are viewed in such a way that they have one dimension of classification. Hierarchies divide and redivide things into groups where each new group is a sub-species of its parent group; everything that is true of a group is also true of its sub-groups and so on down…
The classic example is Linnaeus’ animal kingdom taxonomy. It is classified based on physical attributes. Mammals have hair, give birth to live young. Viviparous quadrupeds are all four legged animals that give birth to live young. Fish are cold blooded, live in the water, and lay eggs. We could, however, re-do this animal kingdom to group species by genetics. This would show a more accurate hierarchical structure, though to normal people, we may not be able to see the relation between items.
If you are a cell phone manufacturer, you might have a class of phone and specific phones within that phone line. For example, Apple has the iPhone with specific iPhone 3G, 3GS, 4, 4S.
Hierarchical taxonomies have implicit relationships. In our phone example, the name iPhone is the parent, the broader term. The iPhone 3G is a child term of iPhone. The iPhone 3GS is a related term to the iPhone 3G.
If you think your subject matter fits well into a taxonomy, use a hierarchical taxonomy. If, however, while you are creating this taxonomy, you find that you are forcing matters and that you are duplicating items everywhere, perhaps think about a faceted taxonomy. When you’re duplicating items in a taxonomy, this shows that your taxonomy is not hierarchical. To take an easy example, say you own a candle and home decor website where you want people to be able to find red candles and red throw pillows. In a hierarchical taxonomy, you could have Things That Make Fire>Candles>Red Candles and Things That Are Comfy>Pillows>Red Pillows. Instead of making your hierarchical taxonomy deeper than it needs to be, turn this into a faceted taxonomy. Colour becomes a facet and Product Type becomes a facet (the products being Candles and Pillows).
When to Use a Faceted Taxonomy
You can use a faceted taxonomy when multiple, similar values can be applied to dissimilar items. To quote Denton again,
Facets will handle three or more dimensions of classification. When, for the purposes of the classification, it is possible to organize the entities by three or more mutually exclusive and jointly exhaustive categories, then facets are probably the appropriate classification. Facets can be used to organize the entire world of knowledge, or the clothes in your cupboard, or anything in between.
If we take a look at Zappos.com, we can see a great faceted taxonomy at work. For their shoes, they use categories like colour, size, width, height, and style to classify their shoes. No one aspect is more important than the other and there is no clear relationship between these aspects.
The interesting challenge with faceted taxonomies, Denton points out, is,
They do not require complete knowledge of the entities or their relationships; they are hospitable (can accommodate new entities easily); they are flexible; they are expressive; they can be ad hoc and free-form; and they allow many different perspectives on and approaches to the things classified. [There are] three major problems: the difficulty of choosing the right facets; the lack of the ability to express the relationships between them; and the difficulty of visualizing it all.
Admittedly, while working on faceted taxonomies, it is difficult to choose the right facets. You need to choose one that is broad enough to encompass the right things, but narrow enough so as not to be all encompassing. There is no explicit relationship between facets and people have a hard time grasping how they will actually work once used “in real life.” You can have a size here, a colour there, a style there, but “How will this look when I search for something? How will I choose these things? How will the database know to combine all these values together? How do I markup my shoes so the database can return the right results?” (And I would point out there that this is where we move from taxonomy to technology…)
Hopefully this gave you a better idea of when to use hierarchical and faceted taxonomy. What are some other pointers that you have? What are some questions that you have?
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Basics of Taxonomy Use and Maintenance
Denton, William. (2009). How to Make a Faceted Classification and Put It On the Web. http://www.miskatonic.org/library/facet-web-howto.html
Nichani, Maish. (2012). Organizing Digital Information for Others. http://pebbleroad.com/perspectives/organizing-digital-information-for-others