Five heuristics for designing classification

Whether you’re designing a home page, a store layout or a document library, you’re practicing information architecture to define a classification scheme. At the heart of this scheme is a set of classes. These act as “shelves” on which you group sets of “books” that have something in common.

For example, the classes “Red, white and rosé” classify wines by colour. “Acoustic, solid electric, semi-acoustic and electro-acoustic” categorize guitars by body style. “Starters, entrées and desserts” classify recipes by course. The wine lover knows to look in “White” for Chablis and Frascati. A guitarist confidently goes to “solid electric” to find a Stratocaster. A chef will expect to find a Pavlova under “desserts”.

An effective scheme reliably lumps similar items together and splits different items apart. Here are 5 heuristics that should help. In each example, misfit classes are highlighted.

Homogenous	The classes describe similar types of things at the same level. Labels use consistent formats and labels.	apples, pears, bananas, sprouts , peaches, Cox’s Pippins, apricots, actinidia deliciosa Sprouts are out of place in the fruit bowl. Pippins are a type of apple, not a type of fruit. Actinidia deliciosa is Kiwi fruit. Although the class fits well, the latin label is inconsistent.
Mutually exclusive	The classes do not overlap. Any item clearly fits in one – and only one – class.	saloons, hatchbacks, coupés, family, estates, sports, 4X4s, vehicles The two lifestyle classes, “family” and “sports”, overlap with the body style classes. “Vehicles” is a superclass of all other classes – and overlaps everything.
Collectively exhaustive	As a set, classes cover all items.	French, German, Spanish, Californian, Chilean Where’s the Valpolicella?
Understandable	Classes have labels that make sense to the reader.	rhinovirus, tussis, cephalalgia Coughs, colds and headaches – if you happen to be a doctor or a classicist.
Useful	The classes group items in a way that supports the reader’s needs.	Red books, blue books, green books, yellow books, grey books, black books, white books Perfect if you want to coordinate your library with the soft furnishings. Otherwise, consistent, logical and useless.

For more on library science, I recommend Classification made simple” by Eric Hunter.