Three-Mile Island! Many people do not know where this place is but they know very well what happened there - the largest nuclear accident in U.S. history. Something went wrong. Nearly very wrong. Why? Obviously some mechanical system failed, but the failure that occurred had been anticipated by the engineers that built the nuclear power plant and should not have been much of a concern. Sophisticated pieces of equipment are designed with the expectation that some time some part of the system will fail. The reason Three-Mile Island became serious was that the information the operators had about the failure was poor and they responded inappropriately. The United States was within about 2 minutes of a disaster of the scale of Chernobyl and yet it was completely unnecessary. Had the computer control room been adequately designed, taking into account human abilities and limitations, the disaster at Three-Mile Island would not even have rated a front page note in the local paper of the area. In this disaster we have one of the defining examples of the need for and importance of human factors.

A good, yet simple, definition of human factors is the design of equipment that people use intentionally taking into account how people operate. Since human beings have characteristic ways of perceiving, thinking, and feeling which cannot be easily modified, it makes sense to change machines to fit us rather than the other way around. Human factors is the scientific/engineering field that collects the relevant data for understanding how humans interact with machines, and uses that information towards the design and implementation of human-machine systems.