Saturday, September 16, 2006

The 90/90 Rule of Design

Or the 95/95 rule, although 90/90 rolls off the tongue more easily.

We have all heard of the 80/20 Rule a.k.a. the Pareto Principle and which comes under a broader category of relationships called the Power Law. In essence the Principle is a rule-of-thumb that states that for many phenomena, 80% of consequences or events are the result of just 20% of the causes. Thus, 20% of scientists are responsible for 80% of all discoveries, 20% of actors are behind 80% of blockbuster movies, 20% of the members of a team do 80% of the work, or 20% of websites get 80% of the hits. 80% is actually only a rough measure. In an actual instance, it could be more like 95/5 or even 99/1 as in the case of weblogs where less than 1% of blogs get greater than 99% of all visitors to blogsites.

The Pareto Principle may be applied to computer applications such as Microsoft Word or Excel: most users employ less than 5% of the applications' features. The counter argument usually advanced is that the 5% of features employed by User A might be different from the 5% of features required by User B. Further, the same user might require a different set of 5% of the features at different times.

Nevertheless, we may find that 95% of the time User A uses the same 5% of an application's features. Extending this logic, if we look at what 95% of all users need 95% of the time, we would discover that most people can get by with a relative small set of features. This Son of the 80/20 Rule we shall hereafter call the 90/90 Rule: the features and functions (f&f) employed by 90% of users 90% of the time.

How dow apply this rule to the design of interfaces? As a general principle, the interface should reveal or provide direct, immediate access to, or control of, only 90/90 f&f; all others should be buried one or more layers below the surface -- in other words, the remaining f&f require the user to perform additional. The user, therefore, is not overwhelmed with interface cues and functions that exceed the 7 +/- 2 chunk cognitive capacity of her Short Term Memory and is able to easily focus on a few primary functions. Additional functions are just a click or two away.

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