Below is a slightly longer discussion of what is known about how physicians'
make probability judgments, with some more references than were in my original
posting

     There is already considerable evidence, some that we have
provided, that physicians may have trouble making diagnostic or
prognostic judgements(1-5)and that individual physicians(6) and
groups of physicians vary in their ability to judge these
probabilities.(7) There are several specific reasons why
physicians may have trouble judging outcome probabilities. These
have to do again with the mental models they use to make
judgements. Judgment and decision-making (JDM) psychologists
have performed experiments to assess how these models are
affected by cognitive biases, how they incorporate mental short-
cuts or rules of thumb, called "judgmental heuristics," and have
developed particular models of judgment processes, such as the
"Brunswik lens model."
     A cognitive bias can be roughly defined as a tendency to
systematically over- or underestimate probabilities of particular
outcomes due to extraneous influences. One bias of interest is
the "value bias:" biasing judgments of the probability of an
outcome according to the importance of the outcome. We have
shown this bias may affect physicians' judgments of the
probability of streptococcal pharyngitis,(5) and of
bacteremia,(8) and Wallsten and colleagues have shown it may
affect physicians' judgments of the probability of having a
urologic tumor.(9)
     Another cognitive bias is ego bias. People may believe the
outcomes of their own actions or of the actions of a group or
institution with which they are affiliated are likely to be
better than average.(10) We have shown the effect of ego bias on
physicians' prognostic judgments.(7) Wright and Ayton have
suggested that ego bias is most likely to affect people who think
they can exert control over the events in question,(11) although
people may have too much faith in their personal ability to
control future events (the "illusion of control").(12) Weinstein
has postulated that unrealistic optimism depends on one's
perceived ability to control the event to be predicted.(13) Ego
bias may also be due to wishful thinking when one is personally
involved.(14)
     There is also evidence that use of potentially misleading
heuristics may degrade physicians' diagnostic and prognostic
judgments.(15) One such heuristic is the "availability
heuristic:" basing judgments of an outcome probability on the
ease with which one can recall instances of similar outcomes.
Since vivid memories may be more easily recalled than mundane
ones, this heuristic could cause one to overestimate the
likelihood of unusual or bizarre events and underestimate the
likelihood of more mundane ones. We have shown the effects of
the availability heuristic on physicians' diagnostic
judgments.(8)
     The Brunswick lens model suggests that people make judgments
by simultaneously assessing the values of several cues (case
characteristics) and then combining them to form a judgment.
This model has been used to study physicians' judgments and
decisions.(16) Problems may arise when people use cues which may
appear to be predictive of the outcome in question, but actually
are not.
     Why many people may pick such non-predictive cues? In a
medical context, the "representativeness heuristic" means making
a probability judgment for a particular case according to its
similarity to a "classic" case in which the outcome of interest
occurred. This could translate into choosing cues based on the
characteristics of classic cases. A related concept is "magical
thinking," defined as assessing the degree of empirical
relationship or correlation among objects or concepts according
to their resemblance or apparent conceptual relationship.(17)
We have shown the effects of the representativeness heuristic on
diagnostic judgments for streptococcal pharyngitis(5) and
Christensen-Szalanski and Bushyhead have demonstrated its effect
on diagnostic judgments of the probability of pneumonia.(18)

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