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Often when creating or analysing an interface design we are faced with the question of whether or not it is intuitive. The creation of an intuitive interface is something we strive for, but beyond a vague sense of making it “easy to use”, is it always clear what we mean by an intuitive design?

Typically, people will rely on some instinctive gut feeling to determine an answer to this question, summoning on experience, knowledge of user behavior and usability best practice. And as useful as these are, this idea of an intuitive design can still seem a bit of a fuzzy concept.

One of the smartest ways in which I've seen this problem addressed was suggested by Jared Spool. He offers a good way of describing this issue, which helps grasp in an informal way, the extent to which a design or interface is intuitive.

He suggests, that in order to successfully interact with an interface a user requires a certain amount of understanding or knowledge. They need knowledge of both the interface and the subject area that the tool works within in order to perform tasks.

So for example, in order to carry out a task in Photoshop, a user is required to understand the interface (e.g. how to open files, use the paint brush, etc.) as well as the subject area (e.g. understand photographs, colours, DPI for print, etc).

These two types of knowledge are called tool knowledge and domain knowledge respectively. Once the user has a sufficient combination of tool knowledge and domain knowledge to carry out task with the interface, they have what Spool calls the required knowledge to use the tool. The concept of required knowledge is the first element of Spool's definition of intuitive design.

The next part of the equation is something Spool calls current knowledge. This is the level of understanding or knowledge that a user possesses when they first start interacting with the interface. The user can start out using the interface with a complete understanding of the tool and domain, or perhaps with absolutely no understanding, most likely however, they will have a level of knowledge that lies on the continuum somewhere between these two extremes.

Using the two concepts of current knowledge and required knowledge we can begin to gauge how intuitive or not an interface is. In particular, if a user's current knowledge is equivalent to the required knowledge then the user will find the interface intuitive, as they will understand how to use it based on what they already know. If, however, there is a knowledge gap between what they know, and what they need to know, then the user has to spend time learning about the tool or domain. The larger the knowledge gap, the more they have to learn, and so the less intuitive the interface is. I've created a diagram to sum this up:

So, returning to our initial question of what makes a design intuitive? A design is intuitive if the knowledge gap between current knowledge and required knowledge is small. The smaller the gap, the more intuitive the interface will be to use.

As with a lot of concepts within usability, this seems really obvious once stated, but usability is the application of common sense and half the battle can be finding the right way to approach a problem. Spool's ideas on interface intuitiveness provide a useful framework for dealing with a vague subject.