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Assessment of decision structure 157
Keeney 15 has fewer reservations:
Often the complex problems are so involved that their structure is not well
understood. A simple decision tree emphasizing the problem structure which
illustrates the main alternatives, uncertainties, and consequences, can usually
be drawn up in a day. Not only does this often help in defining the problem,
but it promotes client and colleague confidence that perhaps decision analysis
can help. It has often been my experience that sketching out a simple decision
tree with a client in an hour can lead to big advances in the eventual solution
to a problem.
Many decision makers report that they feel the process of problem
representation is perhaps more important than the subsequent computations.
Humphreys 16 has labeled the latter the ‘direct value’ of decision
analysis and the former the ‘indirect value’. Decision analysis provides
the decision maker with a
convincing rationale for choice, improves communication and permits direct
and separate comparisons of different people’s conceptions of the structure
of the problem, and of the assessment of decomposed elements within their
structures, thereby raising consciousness about the root of any conflict.
However, some studies have illustrated that the decision makers’
estimates, judgment and choices are affected by the way knowledge
is elicited.
This research has direct relevance for the decision analyst’s attempts
at structuring. In one study, Fischhoff et al. 17 investigated estimation
of failure probabilities in decision problem representations called fault
trees. These fault trees are essentially similar to decision trees, with
the exception that events rather than acts and events are represented.
Figure 6.10 gives a fault tree representation for the event ‘a car fails to
start’. This is the full version of the fault tree that Fischhoff produced
from the use of several car-repair reference texts.
In several experiments Fischhoff presented various ‘full’ and ‘pruned’
fault trees to members of the public. For example, three of the first six
sub-events in Figure 6.10 would be omitted from the presentation to be
implicitly included under the seventh sub-event ‘all other problems’.
Fischhoff asked:
For every 100 times that a trip is delayed due to a ‘starting failure’ estimate,
on average, how many of the delays are caused by the 7(4) factors?
Fischhoff found that the amount of probability placed on ‘all other
problems’ did not increase significantly when it contained three of the