The greatest enemy of the truth is not the lie, but the myth. –John F. Kennedy
Myths not only pervade the realm of folklore and legends, but occur with surprising frequency in technical areas. I would define technical myths as truths that are held without supporting data or analysis, and sometimes in contradiction to supporting data. Their existence is seemingly the result of one of two factors: (1) they provide a simplistic solution to a complex problem, and/or (2) a correlation of events is assigned a causal relationship (e.g., every time the wind is high, the unit trips). A common process control operator myth is that human operators can run a process more efficiently than can a multivariable advanced control scheme.
At Beville Engineering, Inc. we have had to fight the notion since the 1980’s that board operator workload could be established by counting control loops, with 200 loops somehow having been assigned the limit. This, despite evidence that some of the lowest loop count units, such as a delayed coker unit, had the highest workload. Alternatively, many units are successfully operated in which the console operator has a span of control in excess of 300 loops. The bottom line is that no human performance data exists to support such a concept. Further, no theoretical constructs have been presented to explain such a metric of workload/staffing.
A new myth has emerged -- An alarm actuation rate of 10 alarms in ten minutes will overwhelm an operator. The myth appears to have its origin in the EEAMU alarm standard 1. Like the loop count, it provides a simplistic explanation to a complex problem; it just doesn’t have much basis in fact. Human performance research shows the human signal detection threshold to be around 15 to 50 events per minute, far higher than the one per minute being touted as a limit 2.
Why such a huge discrepancy between the research data and the readily-accepted myth? The wide variance is a function of the nature of the event (i.e., the content of the signal) as well as the presentation method. Superfluous alarms, improper prioritization, and poor integration of alarm information within displays all work to propagate the myth. Because the problem is complex and seemingly uncontrollable, using a somewhat arbitrary limit provides a goal to work for…a metric whereby those in process control can determine if their system is acceptable.
Can alarm actuation rates exceed human signal detection limits? Of course they can. At Beville, we use fifteen events per minute as the detection threshold and five events per minute as the comprehension threshold, but we do so with an understanding that these are simplistic rules-of-thumb. They are useful in understanding that the heater trip causing 60 alarms per minute was probably more than the operator could handle.
Why is our data better than the EEAMU data? Ours comes from research published in a peer-reviewed scientific journal; the other is opinion. Will this new myth persist and grow? Only if we all continue to refuse the intellectual challenge to ask for the facts.
1 “Alarm Systems: A Guide to Design, Management and Procurement,” Engineering Equipment and Materials Users Association, London, England, 1999, pp102-109
2 Lanzetta, T., Dember, W., Warm, J. and Berch, D. “Effects of Task Type and Stimulus Heterogeneity on the event Rate Function in Sustained Attention,” Human Factors, Vol 29, No. 6, 1987, pp 625-633. 2
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