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The role of iteration in human-centered design is to enable continual refinement and enhancement. The goal is rapid prototyping and testing, or in the words of David Kelly, Stanford professor and cofounder of the design firm IDEO, “Fail frequently, fail fast.” Many rational executives (and government officials) never quite understand this aspect of the design process. Why would you want to fail? They seem to think that all that is necessary is to determine the requirements, then build to those requirements. Tests, they believe, are only necessary to ensure that the requirements are met. It is this philosophy that leads to so many unusable systems. Deliberate tests and modifications make things better. Failures are to be encouraged—actually, they shouldn’t be called failures: they should be thought of as learning experiences. If everything works perfectly, little is learned. Learning occurs when there are difficulties. The hardest part of design is getting the requirements right, which means ensuring that the right problem is being solved, as  well as that the solution is appropriate. Requirements made in the abstract are invariably wrong. Requirements produced by asking people what they need are invariably wrong. Requirements are developed by watching people in their natural environment.

When people are asked what they need, they primarily think of the everyday problems they face, seldom noticing larger failures, larger needs. They don’t question the major methods they use. Moreover, even if they carefully explain how they do their tasks and then agree that you got it right when you present it back to them, when you watch them, they will often deviate from their own description. “Why?” you ask. “Oh, I had to do this one differently,” they might reply; “this was a special case.” It turns out that most cases are “special.” Any system that does not allow for special cases will fail.

Getting the requirements right involves repeated study and testing: iteration. Observe and study: decide what the problem might be, and use the results of tests to determine which parts of the design work, which don’t. Then iterate through all four processes once again. Collect more design research if necessary, create more ideas, develop the prototypes, and test them.

With each cycle, the tests and observations can be more targeted and more efficient. With each cycle of the iteration, the ideas become clearer, the specifications better defined, and the prototypes closer approximations to the target, the actual product. After the first few iterations, it is time to start converging upon a solution. The several different prototype ideas can be collapsed into one. When does the process end? That is up to the product manager, who needs to deliver the highest-possible quality while meeting the schedule. In product development, schedule and cost provide very strong constraints, so it is up to the design team to meet these requirements while getting to an acceptable, high-quality design. No matter how much time the design team has been allocated, the final results only seem to appear in the last twenty-four hours before the deadline. (It’s like writing: no matter how much time you are given, it’s finished only hours before the deadline.)

Excerpted from ‘The Design of Everyday Things’ by Don Norman