
74 Human factors engineering and ergonomics
point where the two normal curves cross
in Figure 4.3. It is also the
point where the two types of errors have equal probability of accuracy.
Probabilities, as defined in the chart, take into account subjects’ response
behavior under conditions where the noise and signal plus noise are equally
likely. If the experiments were devised so that there was a 90% chance of a
signal being present, a subject might be inclined to respond as if there were
a signal on every trial, thereby obtaining a total accuracy of 90%; errors
would be biased toward false alarms.
Subjects respond dierently, however, when ...