Subjective Scales of Effort and Workload Assessment - Fundamentals and Assessment Tools for Occupational Ergonomics - page 739

TABLE 37.6 The Description of Workload Dimensions of Subjective Workload Assessment Technique (SWAT)
I. Time load
1. Often have spare time. Interruptions or overlap among activities occur infrequently or not at all
2. Occasionally have spare time. Interruptions or overlap among activities occur frequently
3. Almost never have spare time. Interruptions or overlap among activities are very frequent, or occur all the time
II. Mental effort load
1. Very little conscious mental effort or concentration required. Activity is almost automatic, requiring little or no attention
2. Moderate conscious mental effort or concentration required. Complexity of activity is moderately high due to uncertainty,
unpredictability, or unfamiliarity. Considerable attention required
3. Extensive mental effort and concentration are necessary. Very complex activity requiring total attention
III. Psychological stress load
1. Little confusion, risk, frustration, or anxiety exists and can be easily accommodated
2. Moderate stress due to confusion, frustration, or anxiety noticeably adds to workload. Significant compensation is required
to maintain adequate performance
3. High to very intense stress due to confusion, frustration, or anxiety
4. High to extreme determination and self-control required
measurement methodology is applied to develop the best-fitting scale for perceived workload. SWAT has
the capability to account for individual differences by grouping the subjects according to the dimensions
they emphasize most in their ratings (Meshkati et al., 1992). A separate workload scale can be derived for
each subgroup. Although the individuals are not asked to evaluate the importance of each of the three
SWAT dimensions, the estimates of the relative importance are obtained as a function of rescaling of
rank-ordered data by conjoint scaling procedure (Nygren, 1991). The obtained scale is used in the
event scoring phase to assess the workload associated with performed task.
Mental workload assessment with SWAT was extensively tested in diverse environments, including
military flight scenarios and commercial air travel (Nataupsky and Abbott, 1987; Battiste and Bortolussi,
1988), nuclear plant simulations (Beare and Dorris, 1984), military tank simulators (Whitaker, Peters
and Garinther, 1989), different systems of air defense (Bittner et al., 1989), and remote control vehicles
(Byers et al., 1988). SWAT demonstrated sensitivity to variations in mental workload during a variety of
tasks, including visual display monitoring, memory tasks, and manual control (Rubio, et al., 2004). It was
also found that the three SWAT rating scales are differently sensitive to the tasks demands. Therefore,
it was suggested that the individual scales have differential diagnosticity in assessing workload, and
individual scale information should be retained and separately examined as workload components
(Moroney et al., 1995).
37.4.3 Modified Cooper-Harper Scale
The Cooper-Harper scale is one of the first standardized scales for measuring workload, was originally
developed to evaluate handling qualities of the aircraft (Cooper and Harper, 1969). It was concluded that
this technique is well suited to evaluate other manual control tasks as well (Moray, 1982; Skipper et al.,
1986). In order to make the instrument applicable to a wider variety of tasks, the modified scale was
developed (Modified Cooper-Harper: MCH scale) (Wierwille and Casali 1983). The modification
was done in order to assess workload associated with cognitive functions, such as perception, monitor-
ing, evaluation, communications, and problem solving. The flow diagram of the original technique was
retained, but the verbal descriptors and the rating scale range were changed. The MCH scale consists of a
decision tree and a unidimensional 10-point rating scale that ranges from easy (1) to impossible (10).
Figure 37.2 presents the MCH decision tree and rating scale.
The MCH scale is especially appropriate for evaluation of tasks with perceptual, mediational, and
communications activities (Casali and Wierwille, 1983, 1984). It was argued that application of the
decision tree flowchart in the subjective rating scale may reduce the variability due to its tighter structure,
whereas conventional scales such as, bipolar leave too many of the scale levels open to operator judgment
and selection variability (Skipper et al., 1986). However, the decision tree scales can provide only ordinal