stimulus with a value of x larger than 2 comes from the signal distribution
and is given the response Yes. The position of the threshold can also be given
relative to the signal distribution (because the noise has zero mean, B is the
distance of the threshold relative to the noise distribution), as the mean of
the signal is equal to d
0 we can compute D as D = d
0 − B (a value equal to
−1 in our example).
The most popular way of expressing the location of the threshold, however,
is neither from the distribution of the noise nor the distribution of the
signal but relative to what is called the ideal observer. The ideal observer
minimizes conjointly the probability of a Miss and of an FA. When each type
of errors has the same cost, the criterion of the ideal observer is positioned
on the average of the means of the signal and the noise distribution. In our
example, the threshold of the ideal observer would be equal to 1
2
d
0 =
1
2 = .5.
The value of C is the distance from the actual threshold to the ideal observer,
it can computed as C = B −
d
0
2 = 2 − .5 = 1.5. The sign of C reveals
the participant’s strategy: when C = 0, we have the ideal observer; when
C is negative the participant is lib´eral (i.e., responds Yes more often than
Table 2: The probability or the four possible types of response according to
to Figure 2.