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dimensions described in multiple resource model of Wickens (Rubio et al., 2004). The workload dimen-
sions represent demands that can be imposed by a task, including perceptual
central processing,
response selection and execution, spatial processing, verbal processing, visual processing, auditory
processing, manual output, and speech output. This instrument aims to combine the advantages of sec-
ondary task performance based procedures (high diagnosticity) and subjective techniques (high subject
acceptability and low implementation requirements and intrusiveness) (Rubio et al., 2004). During
workload assessment with WP, the subjects are asked to provide the proportion of attentional resources
used after experiencing all of the tasks to be evaluated. Subjects are provided with the definition of each
dimension at the time of their rating. Each dimension is rated with the number between 0 and 1 to reflect
the proportion of attentional resources used in each task. A rating 0 means that tasks placed no demands
on the dimension rated, and 1 means the maximum attentional demands (Tsang and Velazquez, 1996).
The WP procedure performance was investigated by two studies (Tsang and Velazquez, 1996; Rubio
et al., 2004). Tsang and Velazquez (1996) established instrument reliability with test-retest method
and concurrent validity in reference to task performance. Both studies showed that WP ratings are
sensitive to the task demand manipulations. However, the properties of WP demand more detailed
and extensive research (Tsang and Velazquez, 1996; Rubio et al., 2004).
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37.4.5 Other Methods of Mental Workload Assessment
37.4.5.1 Overall Workload Scale
The Overall Workload (OW) scale is a bipolar scale requiring subjects to provide a single workload
rating. The OW scale is easy to use, but is less valid and reliable than NASATLX or AHP ratings (Vidulich
and Tsang, 1986). Hill et al. (1992) reported that OW was consistently more sensitive to workload and
had greater operator acceptance than the MCH rating scale or the SWAT.
37.4.5.2 Bedford Scale
The Bedford scale is a unidimensional rating scale designed to identify operator's spare mental capacity
while completing a task. The Bedford scale is a modification of the Cooper-Harper scale (Rehman,
1995). The single dimension is assessed using a hierarchical decision tree that guides the operator
through a 10-point rating scale, each point of which is accompanied by a descriptor of the associated
level of workload. It is simple, quick, and easy to apply in situ to assess task load in high workload
environments, but it does not have a diagnostic capability. The Bedford scale provides a good
measure of spare capacity (Tsang and Johnson, 1987), and demonstrates high sensitivity to the manip-
ulations of task demands (Tsang and Velazquez, 1996). However, other research showed that the scale is
not sensitive to differences in either control configurations or combat conditions (Rehman, 1995).
37.4.5.3 Defence Research Agency Workload Scale
The Defence Research Agency Workload Scale (DRAWS) (Table 37.7) is a multidimensional tool
designed to perform subjective assessment of the operators' workload. This instrument evaluates work-
load along several dimensions, including input demand, central demand, output demand, and time
pressure. Input demand is a load associated with the acquisition of information from external
sources, central demand is the load associated with interpreting information and deciding on action.
TABLE 37.7 Defence Research Agency Workload Scales (DRAWS)
(a) How much demand was imposed by the acquisition of information from external sources (e.g., from a visual display or
auditory signals)?
(b) How much demand was imposed by the mental operations (e.g., memorization, calculation, decision making) required
by the task?
(c) How much demand was imposed by the responses (e.g., keypad entries, control adjustments, vocal utterances) required
by the task?
(d) How much demand was imposed by time pressure?
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