design patterns - What objective qualities make a user interface intuitive?

How is intuitive defined in terms of UI? Are there any studies about what objective and measurable qualities make a user interface intuitive? What design principles, patterns, or approaches are most likely to produce a UI that is considered intuitive?

Answer

Good question. Wikipedia lists intuition as "thoughts and preferences that come to mind quickly and without much reflection" - so basically, saying a UI is intuitive is like saying it exhibits several positive attributes: it's memorable, discoverable, easy to learn, familiar, matches expectation, and so forth. But let's not take my word for it. Let's refer to the experts!

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  Jef Raskin wrote the definitive article about intuitive interfaces in 1994. In it, he inspects several quotes discussing intuitive interfaces or intuition, and remarks on what they implied. "When the tools had been learned, [...] they became intuitive. This is a strong clue as to the meaning of 'intuitive'," he says about the author of a a review he read (and then proceeds to reference Star Trek IV). Later, he concludes that "'intuitive' in [a certain] context is an almost exact synonym of 'familiar'". And finally, he arrives at a definition: "Intuitive = uses readily transferred, existing skills."

  
  


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  Jared Spool also wrote about intuitive interfaces in an article in 2005 called "What makes a design seem 'intuitive'?". One interesting thing he points out is that "interfaces can’t be intuitive, since they are the behavior side of programs and programs can’t intuit anything. When someone is asking for an intuitive interface, what they are really asking for is an interface that they, themselves, can intuit easily." Afterwards, he introduces the concept of the Knowledge Gap, which is the difference between what the user knows and what the user needs to know in order to understand how to use the software:

  
  
  

  

  
  

  An intuitive interface, he argues, will bridge that gap. He identifies two separate conditions for intuitive interfaces. In the first case, the knowledge gap doesn't exist because the user already possesses all the knowledge required to use the interface. In the second case, the user doesn't notice the knowledge gap since the software is training them to use it.

  
  

  Finally, he suggests what is needed to design an intuitive interface: "What do users already know and what do they need to know? To build intuitive interfaces, answering these two questions is critical." For the first, he recommends field studies and for the second, he recommends usability testing.

  
  


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  In psychology, the Exposure effect "is a phenomenon by which people tend to develop a preference for things merely because they are familiar with them". This phenomenon was first researched in 1876 by Gustav Fechner and likely has a large impact on how people perceive interfaces from a familiarity standpoint, which affects how they intuit them.