COGNITIVE AND AFFECTIVE OBSTACLES
All people have trouble drawing valid conclusions from a small
sample; even professional psychologists with statistical training
tend to overgeneralize and to let false intuition get ahead of formal
statistical procedures (Tversky and Kahneman, 1971; Kahneman
and Tversky, 1973). No wonder that a hurricane-zone shopkeeper
who has seen only one severe hurricane in 20 years has difficulty
making an estimate of the likely future occurrence of damaging
winds. Few geophysical records are available for longer than 60
years, but the estimated recurrence intervals of damaging events
may run from 25 to 1000 years.
As Slovic et al. have pointed out (1974), some people fall into
what is called the "gambler's fallacy," some tend to rely unduly
upon the evidence of a limited but concrete sort, and others follow
a conservative "anchoring" procedure. "me "gambler's fallacy' as-
sumes that if an event has occurred one year, it is less likely to
occur the following year. The hypothetical Caribbean fisherman of
Chapter 2 was assumed to increase his probability because of re-
cent exposure. Hydrologists know that if the event is random the
probability of its recurring is just the same one year as the next.
Two floods estimated to have a probability of once in a century
took place on the Housatonic River (New England) during the
same summer of 1955. Another flood might have come the follow-
ing year—or not again for two centuries. For extreme geophysical
events that are not random—such as earthquakes—the analysis is
different, since the severe occurrence results from accumulated
strains that take time to build up again once they have been re-
lieved. But for such events as floods and heavy snowfall, people
have a tendency to assume that one severe occurrence heralds a
period of respite. This was the case for LaFollette (Tennessee)
floodplain dwellers (Kates, 1962).
In frying to interpret information about past events, people also
seem inclined to use a concrete piece of information—perhaps from
a neighbor—as being representative of a larger, more abstract,
body of information. The Australian farmer is exposed to a large
body of meteorological and climatological data when he ap-
proaches the crucial decision of when to seed his crop—and then
proceeds to use a reading from his rain gauge as a basis for choice;