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Sustaining Vigilance - Optimizing The Lower End Of The Workload Scale


It is 3 A.M. and the chemical process is humming along thanks to the advanced control system. Unfortunately, the operator sitting at the console is sound asleep. Since the advanced control system was working so well the operator had little to do, became bored, and fell asleep. Does this scenario sound familiar?

Although advanced process control has improved the efficiency of chemical processing units, it has also created some real problems with vigilance. When the advanced control systems are running well, there are few, if any, manual tasks the operator must perform other than scan the displays and maintain their awareness of process conditions. Human performance follows a bell shaped curve; too many demands on a person and they become overwhelmed and overloaded, too few demands and they become bored, lose their vigilance, and fall asleep. The best performance is achieved at moderate levels of stimulation and workload. The question is, what level of stimulation or interaction with the process do operators need to maintain high vigilance and their awareness of process conditions, and keep them from falling asleep at 3 A.M.?

Research is being conducted to help answer the vigilance questions. One recent study; Methot, L., Huitema, B. (1998). Effects of Signal Probability on Individual Differences in Vigilance, Human Factors, 40, 111-125, investigated the effects of signal probability on individual differences in vigilance performance. The authors were trying to find a level of stimulation that is needed to ensure all persons maintain high levels of performance in a vigilance task.

In the experiment, subjects monitored a display containing two simple gauges with normal zones, danger zones and arrows that would change positions every 1.5 seconds. The subjects were to detect every time the arrows went into the danger zones. The researchers found that there is a lot of variation between subjects, and to get all subjects to perform at high levels of performance, the signal probability had to be a relatively high 12%, or about five signals per minute.

Obviously, operators do not continuously monitor their displays and five signals, or alarms, per minute for extended periods would cause most operators to look for employment elsewhere. Beville believes that in the context of a chemical plant control board position, where operators have alarm systems, take breaks from monitoring, and complete other tasks, the needed level of stimulation is somewhat different. Based upon information collected through Bevilleís control board position benchmarking technique, Beville believes a target level of stimulation for control board operators is approximately twenty tasks per hour with one minute between tasks. The results of the study highlight the importance of designing the proper amount of stimulation into the control board operatorsí job. Too much or too little activity will degrade their performance.

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For further information, visit the links below.

| Workload/Staffing Analysis Overview | Steady State Workload Staffing Analysis | Steady State Job Samples | Upset Response Staffing Analysis | Overtime Calculator -Excel download |


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This year's Fall meeting for the Center for Operator Performance will be October 7-10 in Houston, TX. For more information, please contact Lisa Via. Guests are always welcome!


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David Strobhar's book, "Human Factors in Process Plant Operation," is now available in both hardcover and Kindle e-book.

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