I have reconsidered some most cherished beliefs in the practice of design and found many of the principles wanting. We know surprisingly little about how to do design. There is no science of the practice in the same sense that there is a science to the structural analysis of buildings and bridges, or to the building of circuits. Design is still an art, taught by apprenticeship, with many myths and strong beliefs, but incredibly little evidence. We do not know the best way to design something. The real problem is that we believe we do. Beliefs are based more on faith than on data.

This is a problem that confronts all professional disciplines: law, art, music, business, medicine, and design. Each of these disciplines often has some scientific field behind it (e.g., art and music has perceptual psychology, interaction design has well-established psychological roots, many parts of business have a basis in decision theory, economics, and finance, and medicine has biology and chemistry. But even in the fields with a substantive scientific basis, the practical applications to the daily practice are very limited. Thus, although biology is important as a foundation for medicine, it gives no guidance to patient-doctor interaction, to the taking of patient histories, or to diagnosis, nothing to say about patient empathy or best hospital practices. In business, finance and economics provide a rationale for some kinds of investment decisions, but where do best management principles come from? In law, what science underlies jury selection or presentations? Music has lots of theory, but very little is directly relevant to music performance. In the end, practical disciplines are all taught by apprenticeship, internships, residencies, and long periods of training.

In science, there are clear links among hypotheses, conclusions, and evidence. But in the practices of most professions, the links are tenuous at best. Instead, there is much reliance upon "best practice," where "best" is often defined by short-term measurements, usually of variables that are easy to measure as opposed to those that are of most significance. Long-term measures are seldom taken. Methods are seldom compared... Scientists usually operate in what has been called "white room" conditions, carefully forming abstract characterizations of the phenomena under consideration and studying them in a controlled research environment or the clean precision of the laboratory. Similarly, the theories are of necessity simplified and abstracted to a pristine form of mathematical or simulation models. Science works best when all the variables are understood and controlled. The real world is complex and messy, with uncontrolled variables, sometimes behaving in ways that contradict the neat, tidy, logical assumptions of the scientist. No wonder there is a gap.

The lack of scientific studies of practice is due to two things: First, practitioners are not trained in scientific research. They do not understand the need for experimental controls nor do they understand statistical variability and experimental biases. Moreover, they don't wish to: they want to get on with their work. Second, even when researchers well versed in experimental methods attempt to study practices, they discover that the very nature of a practical discipline throws in so many idiosyncratic variables that rigor is simply not possible.