Under such complexity it is
imperative to know whether or not the mental workload of the operator is too
great for safety. Human-machine systems engineers have sought to develop
measures of mental workload, the idea being that as mental load increases, the
risk of error increases, but presumably measurable mental load comes before
actual lapse into error.
Three approaches have been
developed for measuring mental workload:
1. The first and most used is the
subjective rating scale, typically a ten-level category scale with descriptors
for each category from no load to unbearable load.
2. The second approach is use of
physiological indexes which correlate with subjective scales, including heart
rate and the variability of heart rate, certain changes in the frequency
spectrum of the voice, electrical resistance of the skin, diameter of the pupil
of the eye, and certain changes in the evoked brain wave response to sudden
sound or light stimuli.
3. The third approach is to use
what is called a secondary task, an easily measurable additional task which
consumes all of the operator’s attention remaining after the requirements of
the primary task are satisfied. This latter technique has been used
successfully in the laboratory, but has shortcomings in practice in that
operators may refuse to cooperate.
Such techniques are now routinely
applied to critical tasks such as aircraft landing, air traffic control, certain
planned tasks for astronauts, and emergency procedures in nuclear power plants.
The evidence suggests that supervisory control relieves mental load when things
are going normally, but when automation fails the human operator is subjected
rapidly to increased mental load.
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