System Safety for the 21st Century, 2nd Edition

CHAPTER 13Human Factors

HUMAN RELIABILITY

A great deal of progress has been made toward improving and evaluating the reliability of hardware systems; however, the place where systems most frequently fail is in the interface of humans with the system. Human reliability is generally much lower and more difficult to control than hardware reliability.

If a stimulus is provided to a piece of hardware, the same response is obtained from that piece of hardware day after day, year after year, until it fails. Even when hardware fails, the failure modes, rates, frequencies, and effects can be predicted with a reasonable degree of accuracy.

People, however, are much more difficult to analyze. A given stimulus provided to a person or group of people may produce entirely different responses over time because of the mediation process that takes place (Fig. 13‐1). Individuals judge the input stimulus based upon highly individualized value systems. For this reason, accurately determining what type of response will result from a given stimulus is very difficult. Not only does this response vary from group to group and from individual to individual, but it also varies with the same individual over time. A response today may be totally different from the response to the same stimulus three days from now.

Stress Resistance and Failure Modes

Humans also vary widely in stress resistance, both physiological and psychological. Suppose a specific amount of force is applied in a given direction, at ...