Thinking and Deciding

138 DESCRIPTIVE THEORY OF PROBABILITY JUDGMENT
Figure 6.1: Subjects’ estimates of the frequency of various causes of death as a function
of actual frequency. From S. Lichtenstein, P. Slovic, B. Fischhoff, M. Layman,
and B. Combs, “Judged frequency of lethal events,” Journal of Experimental Psychology:
Human Learning and Memory, (1978): 4, 565. Reprinted by permission of
the first author, and of M. G. Morgan and M. Henrion, who redrew the figure.
perience with, they underestimate very high frequencies and overestimate very low
ones. For example, Attneave (1953) asked subjects to estimate the relative frequency
with which various letters of the alphabet occur in ordinary written English. What
percentage of all of the letters in a newspaper, for example, are as, es, and so forth?
Subjects tended to overestimate low probabilities (zs) and underestimate relatively
high ones (es). A similar effect was observed by Lichtenstein, Slovic, Fischhoff,
Layman, and Combs (1978) when subjects were asked to judge the frequency with
which various dangers caused deaths in the U.S. population today. The actual frequencies
range from a death rate of 0 for smallpox, to heart disease, which has a rate
of 360,000 per 100,000,000 people per year. A plot of the data from this study is
showninFigure6.1. 2
2 Geometric means of subjects’ responses are used in this graph in order to avoid undue influence
by subjects who gave very high numbers. Subjects were given the correct answer for “motor vehicle
accidents” as a standard.