AAWSAP DIRD, Cognitive Limits on Simultaneous Control of Multiple Unmanned Spacecraft, December 2010

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Chapter 2: Measurement of Mental Workload
There are three approaches to measuring mental workload in a subject performing a
primary task. The first is subjective evaluation, either post-hoc self-report
questionnaires concentrating on how "busy" one may have felt, or an experimental
observation of activity. The second is performance measures, an objective evaluation of
how well the subject completes the primary task, a secondary task, or an
experimentally inserted reference task. The final approach is to record real-time
physiological measures, with the assumption that increased workload increases anxiety
and this will be exhibited by changes in the autonomic nervous system (ANS).
SUBJECTIVE MEASUREMENTS
Self-report measures are appealing because they get inside the mind that was
performing the task. There is no absolute objective scale to measure one person's "fully
occupied" from another person's view of the same state; however, through rating
scales and self-drawn graphs, one can obtain an accurate picture of how perceived
workload evolved during the experiment. Perceived workload is important because it is
what needs to be maintained between a subjective minimum, where attention may
wander, and a subjective maximum, where increased emotion may decrease
performance capacity.
The most frequently used standardized self-report tools are the NASA Task Load Index
(NASA-TLX or just TLX) and the Subjective Workload Assessment Technique
(SWAT) .1,2,3.4 The TLX is a subjective workload assessment based on a multi­
dimensional rating questionnaire. An overall workload score is derived based on a
weighted average of ratings on six subscales: mental demands, physical demands,
temporal demands, own performance, effort, and frustration. SWAT is a two-step
assessment of three workload factors: time load, mental effort load, and psychological
stress load. In the first step, hypothetical activities are ra nked according to perceived
workload. In the second step, the experimental task is evaluated post-hoc, using a 1-3
rating scale for each of the three dimensions. An interval scale of workload is derived,
from 0-100, based on the reference data collected for each subject in the first step, and
the evaluation of the experimental task. A custom self-report can also be designed by
the experimenter to specifically focus on the research questions in a given experiment.
The second type of subjective measure is evaluation by an expert observer. In this type
of measurement, assumptions are made on the mental activity of the subject based on
the activities being performed. The advantage of this approach is that there is minimal
variance per-to-person if the same evaluator is employed. The disadvantage of this
approach is the outside observer can miss workload multiplying factors such as task
complexity contributing to an overwhelmed feeling by the subject. An example of this
approach is the concept of utilization. In ca lculating utilization it is assumed that the
subject must cognitively address one issue at a time in serial order. The measure of
utilization is percent time busy, or addressing any issue, as opposed to waiting or
monitoring for the next event. In general is it observed for control and supervisory
tasks that at around 70% utilization performance begins to degrade. 5 Arguably not a
perfect measure of workload, utilization has the advantage of simplicity, objectivity,
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