Know thyself. ?span>Inscription at the Delphic Oracle, from Plutarch, Morals Use a machine or a tool in accord with its strengths and limitations, and it will do a good job for you. Design a human-machine interface in accord with the abilities and foibles of humankind, and you will help the user to not only get the job done but also be a happier, more productive person. Design guidelines for products that interact with us physically are reasonably straightforward. The sizes and capabilities of the human frame and senses have been well cataloged; these studies form the science of ergonomics. Chairs, tables, keyboards, and displays can be designed with a high degree of likelihood that they will work reasonably well for their human users, although thorough testing can never be neglected. You would not design a machine that required one person to simultaneously operate two switches 3 meters apart: We all know that humans are not that large. Mayhew (1992, Chapter 12) discusses computer-relevant ergonomics, a topic outside the scope of this book, in her overview of interface design. Ergonomics takes into account the statistical nature of human variability. You might design a car seat to accommodate only 95 percent of the population, even though you know that 5 percent of the potential car purchasers will find the seat uncomfortable. It might be too expensive or mechanically impossible to give the seat the range of adjustment needed to work with the rare 1-meter midget or the rarer still 2.5-meter giant. For the most part, the machines that our civilizations have built have been mechanical and have interacted principally with our physical selves. Consequently, our physical limitations are relatively well understood. Increasingly, inventions have come to the aid of intellectual rather than physical pursuits. We must master an ergonomics of the mind if we want to design interfaces that are likely to work well. As surprising as it may seem, we are often blind to our own mental limits; we must rely on careful experiment and observation to discover the edges of our own mind's abilities. The study of the applicable, engineering scope of our mental abilities is cognitive engineering, or cognetics. Certain cognetic limitations are obvious: You do not expect a typical user to be able to mentally multiply a pair of 30-digit numbers in 5 seconds, and you would not design an interface that requires such an ability. But we are often not aware of other mental limitations that adversely affect our performance when we use human-machine interfaces, although these limitations are inherent in every human. Remarkably, all of the well-known computer interfaces and many noncomputer human-machine interfaces are designed as though their designers expect us to have cognitive abilities that experiment shows we do not possess. Much of the difficulty that we have with computers and related devices is due to poor interface design rather than to any complexity inherent to the task or to any lack of effort or intelligence on the part of users. Just as ergonomics takes into account the statistical nature of human variability, so too should cognetics. However, because there has been so little practical use of what is known of the limits of human cognition that are common to all of us, it seems wise to look first at those limits. Fortunately if only because our present knowledge of these topics is uncertain we do not need to examine the structure of the physical brain. We can design successful interfaces based on a pragmatic and empiric view of what the human mind can and cannot do, of how long the mind and body take to do particular tasks, and of the circumstances that increase the likelihood that we will make mistakes. | 
