Thinking and Deciding

384 QUANTITATIVE JUDGMENT
Table 15.3: Case-study data for judgment experiment (1 = symptom present, 0 =
symptom absent)
Case Swollen Splotches Discolored Nosestudy
eyelids on ears gums bleed
R. C. 1 1 1 1
R. M. 1 1 1 1
J. J. 0 1 0 0
L. F. 1 1 1 1
A. M. 1 0 1 1
J. S. 1 1 0 0
S. T. 0 1 1 1
S. E. 1 0 0 0
E. M. 0 0 1 1
Source: D. Medin, M. W. Altom, S. M. Edelson, and D. Freko, “Correlated symptoms
and simulated medical classification,” Journal of Experimental Psychology:
Learning, Memory, and Cognition (1982), 8, 39.
After studying these cases, subjects were asked to classify new cases. In this
particular experiment, the subjects were asked to say which of two matched cases
was more likely to have burlosis. For example, the subject might be given the Cases
1101 and 0111. (The order of the ones and zeros corresponds to the order of the four
symptoms in the table. Thus the first case has the first two symptoms and the fourth,
but no discolored gums.) In these two cases, the second case preserves the correlation
between the third and fourth symptoms, and subjects generally said that this person
was more likely to have the disease. (This case matched one of the original cases, a
person with initials S. T. A subsequent experiment showed that subjects are sensitive
to correlated symptoms even without such matching.)
Putting this result in the context of this chapter, it appears that these subjects were
sensitive to an interaction between two variables. When discolored gums are present,
the cue value of nosebleed is positive; patients with nosebleed are more likely to have
the disease than those without it. When discolored gums are absent, the opposite is
true; patients with nosebleed are less likely to have the disease than those without
it. This is an interaction. Moreover, the results indicate that subjects are sensitive
to the interaction. This is an exception to the more usual finding that judges are not
sensitive to the interactions present.
One explanation of this finding is that subjects classified the new cases on the
basis of their similarity to the cases they had seen. Read (1983) found that social
judgments were often made on the basis of similar cases, especially when no general
rule had been formulated by the person making the judgment. Read asked subjects