ABSTRACT
In all industries, equipment failures and errors by operators and maintenance workers are recognized as major causes of accidents, and much thought has been given to ways of reducing them or minimizing their consequences.2 However, it is difficult for operators and maintenance workers to keep up an error-free performance all day, every day. We may keep up a tip-top performance for an hour or so while playing a game or a piece of music, but we cannot keep it up continuously. Designers have a second chance, opportunities to go over their designs again, but operators and maintenance workers do not get second chances. Plants should therefore be designed, whenever possible, so that they are user-friendly, to borrow a computer term, and can tolerate departures from ideal performance by operators or maintenance workers without serious effects on safety, output, or efficiency.
