Thinking and Deciding

154 DESCRIPTIVE THEORY OF PROBABILITY JUDGMENT
Battery
charge
too low
Faulty
ground
connections
..........
Terminals
loose or
corroded
..........
Battery
weak
Starting
system
defective
Switches
defective
..........
Transmission
not in
“park”
..........
Seat belt
problem
..........
Faulty
starter
motor
..........
Starter
drive
defective
Fuel
system
defective
Insufficient
fuel
..........
Excess
fuel
(flooding)
..........
Defective
choke
..........
Defective
air filter
Ignition
system
defective
Coil faulty
..........
Distributor
faulty
..........
Spark
plugs
defective
..........
Defective
wiring
between
components
Other
engine
problems
Oil too
thick
..........
Pistons
frozen
..........
Poor compression
Mischief
or
vandalism
Theft or
breakage
of vital
part (such
as battery)
..........
Siphoning
of gas
..........
Disruption
of wiring
All
other
problems
Each of the subpossibilities listed can be further subdivided. For example, “faulty
ground connection” can be subdivided into “paint,” “corrosion,” “dirt,” and “loose
connections.” Fischhoff, Slovic, and Lichtenstein (1978) presented this “tree” (with
lines between each main possibility and its sub-possibilities, and with all of the subsub-possibilities),
to a group of automobile mechanics and asked for probability estimates
for the major branches (such as “Battery charge too low”). Subjects assigned a
mean probability of .078 to the last category, “All other problems.” A similar group
of mechanics was then given the tree with two of the branches pruned, those involving
the starting and ignition system. The subjects using the unpruned tree had
assigned mean probabilities of .195 and .144 to these two branches, respectively. In
the pruned tree, the problems that these branches represented should go under “All
other problems,” and the probability for that category should therefore increase from
.078 to .078 + .195 + .144, or .468. The actual mean probability assigned to “all
other problems” by the group with the pruned tree was .140, a slight increase but not
big enough.
When something is not represented in our analysis, we tend not to think of it.
This effect may weigh on the other side of the debate about nuclear-power plants
from the other effects we have been considering. Availability of frightening news
stories, such as those about the Three Mile Island and Chernobyl accidents, as well
as overestimation of the frequency of such accidents (which very rarely occur), may
tend to lead to overestimates of risk, but failure to consider what information might
be omitted from the analysis may lead to underestimates. Most nonexpert estimates
of accident risk are based on just this sort of analysis. People tend to underestimate
the probability of “All other problems.” 5
5 A well-known professor of decision theory — after lecturing on fault trees for cars not starting —
went to the parking lot to find that his own car would not start. After checking all the branches of the