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278 Methods for eliciting probabilities
Preparing for probability assessment
Because of the danger of bias in probability assessment it is a good
idea for the analyst to prepare the ground carefully before eliciting the
probabilities themselves. Spetzler and Stäel von Holstein 1 recommend
that the interview carried out by the analyst should involve three
phases before the probabilities are quantified: motivating, structuring
and conditioning.
Motivating
This phase is designed to introduce the decision maker to the task of
assessing probabilities and to explain the importance and purpose of
the task. Sensitivity analysis should be used by the analyst to identify
those probabilities which need to be assessed with precision. At this
stage the possibility that assessments may be subject to deliberate biases
should be explored (e.g. a manager may overestimate the probability
of high costs in the hope that when costs turn out to be low he will
be seen in a good light). Deliberate bias is, of course, an undesirable
characteristic of an input to an analysis whose intended outcome is
improved decision making.
Structuring
In the structuring phase the quantity to be assessed should be clearly
defined and structured. For example, asking the decision maker vague
questions about ‘the value of shares in the USA in 2004 ′ is unlikely to
lead to reliable responses. ‘The value of the Dow Jones index at the end of
trading on Thursday 1 July 2004 ′ is obviously a less ambiguous quantity.
It is also important at this stage to agree on a scale of measurement
which the decision maker feels comfortable with: if he thinks of sales in
terms of ‘numbers of boxes sold per week’ it would be inappropriate to
force him or her to assess a probability distribution for ‘the number of
tons sold per week’.
When the decision maker thinks that the quantity to be assessed
depends on other factors it may be simpler to restructure the assessment
task, possibly by making use of a probability tree (see Chapter 4 and the
last section of this chapter). For example, it may be that the development
time for a new product will depend upon whether two companies can
agree to collaborate on the basic research. In this case the decision maker