Computer-controlled process plants are sometimes equipped with alarm systems. The purpose of these systems is to warn the operator of abnormal plant conditions in order that he may take remedial action or shut down the plant. The systems are based on programs which compare the values of process variables with pre-set alarm limits. Abnormal conditions are usually reported to the operator by means of a special teletype. On most plants, the operator is then expected to analyse the pattern of alarms and thereby diagnose the cause of the trouble. The time available for this diagnosis may be extremely limited and the penalty for maloperation may be high. The operator is therefore under considerable stress.