Civil Engineering Reference
In-Depth Information
measures some of the stress in traffic
but not all. In addition to reacting to
traffic stress, heart rate also responds
to other parameters, such as physical
work in steering the car and many
other stressors in life that the driver
happens to think about.
Construct Validity
In this case we need to know if the measure addresses an underlying construct or theory.
Many research studies build upon a theory, and develop assumptions and hypotheses
about possible events. Let's assume that we would like to measure the physiological
stress of operators working in a call center. This is considered a stressful job, as the
operators are selling services to potential customers who may not be interested; instead
they can become annoyed at the operators. To measure the stress level, one can opt to
analyze the concentration of stress hormones, such as adrenaline and noradrenalin in
samples of saliva, urine, or blood. The question, then, is to what extent stress hormones
can measure stress at work and to what extent they may be related to a theory of stress at
work. This is illustrated in Figure 3.2.
Let's assume that to enhance traffic safety, it is important to identify the high peaks of
driver stress and then to redesign parts of the roadway so that they are less demanding
and reduce driver stress. The theory or construct is based on Yerkes-Dodson Law. This
law implies that if there are situations where most people have a high stress level, then
there is also a chance that the performance level is reduced. There is an optimal stress
level where there is a chance that performance is greater.
SENSITIVITY
In addition to reliability and validity, a dependent variable must also have sensitivity, so
that one can distinguish between different levels of the independent variables. Assume
that we would like to find a good measure of mental workload in driving. It should have
the sensitivity to distinguish between situations such as driving on a freeway, entering the
freeway, and exiting from a freeway. One type of measure is the capacity a driver has to
perform two tasks at the same time, or dual-task capacity. To investigate these types of
problems Michon (1967) used a tapping task. The idea is that the driver can establish an
even rhythm in verbal tapping—saying “ta-ta-ta” at a rate of one “ta” per second. But
when traffic conditions become difficult the rhythm becomes irregular, because there is
not enough mental capacity for both driving and tapping. If this particular measure can
distinguish between the levels of difficulty in of driving on different road or traffic
environments, then it is a sensitive measure.
