Subjective Scales of Effort and Workload Assessment - Fundamentals and Assessment Tools for Occupational Ergonomics - page 733

and have concurrent validity with performance measures. In most applications, the rating scales are
completed after the task performance and, therefore, are considered as nonintrusive methods.
Furthermore, subjective methods have high subject acceptance and are easy to implement.

37.2 Psychophysical Scaling

Psychophysics is the study of the relationship between the physical qualities of a stimulus and the per-
ception of those qualities (Stevens, 1974). The psychophysical approach assumes that humans are able
to perceive the strain generated in the body by a given work task and to make absolute and relative judg-
ments about perceived effort (Kroemer, et al., 1994). Subjective measurement techniques rely on an indi-
vidual's ability to relate their sensations to some quantitative measure (Noble and Robertson, 1996).
Psychophysical scaling is a technique which allows a subject to assign a numerical value to the subjective
magnitude of a stimulus.
Weber in 1838 and Fechner in 1860 established formal relationships between physical stimulus and its
perceptual sensation (Kroemer et al., 1994). The Weber-Fechner Law states that a relative increase in
external energy corresponds to a fixed increment in subjective intensity (Stevens, 1974). Thus this law
showed that the relationship between the stimulus and the sensation can be defined by a constant,
and equal physical ratios produce equal psychological differences. According to Fechner, a sensation
grows as a logarithm of the stimulus:

S ¼ k log I

where S is the strength of sensation, k is a constant, and I is the intensity of the stimulus.
Stevens (1974) argued that the approach of Fechner is “indirect” because it defines ability to discriminate
among stimuli as the basic unit of sensation. Stevens developed a ratio scaling technique, for example,
magnitude estimation (ME), where the subjects were asked to “directly” assign numbers to stimuli to rep-
resent the magnitudes of their sensations. Stevens (1974) modified the Fechner Law and discovered more
general psychophysical law which holds in all sense modalities. Stevens's power law states that the strength
of sensation (S) and the intensity of its physical stimulus (I) is related by the power function:

I n S ¼ k

where S is the strength of sensation, k isaconstant,I is the intensity of the stimulus, and n is the slope of the
line that represents the power function when plotted on log-log coordinates. According to the
aforementioned power law, equal stimulus ratios produce equal sensation ratios.

37.3 Scales of Perceived Physical Effort

37.3.1 Borg's Scales for Perceived Exertion
Borg (1982) developed formal techniques to rate the perceived exertion associated with different kinds of
efforts. He found that the perception of both muscular effort and force obey the psychophysical function,
where sensation magnitude (S) grows as a power function of the stimulus. Contrary to the Sevens
approach, according to which rating scale should be a “ratio scale,” Borg developed a “category scale”
for rating of perceived exertion (RPE). According to Borg (2001), the category scale takes into
account interindividual subjective differences, which was not possible with conventional ratio scaling
method. Category scales are equally partitioned continuous rating scales that designate categories with
adjectives or a finite set of numbers (Noble and Robertson, 1996). Scales with ratio properties require
individuals to estimate subjective intensities on a scale with an absolute zero and equivalent scale
steps. This is in contrast to the interval scale, which has equal steps but no true zero (Borg, 2001).