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284 Methods for eliciting probabilities
Number of requests Length of line Probability
0 2 2/36 = 0.06
1 6 6/36 = 0.17
2 10 10/36 = 0.28
3 8 8/36 = 0.22
4 5 5/36 = 0.14
5 2 2/36 = 0.06
6 2 2/36 = 0.06
7 1 1/36 = 0.03
36 1.00
The method of relative heights can also be used to assess probability
density functions for continuous distributions. In this case the analyst
will normally elicit the relative likelihood of a few values and then fit a
smooth pdf curve across the tops of the lines.
A comparison of the assessment methods
Which method of probability assessment is the best? A number of
experiments have been carried out to compare the methods, but these
have not identified one single best method (see, for example, Seaver
et al. 5 or Wallsten and Budescu 6 ). Indeed, the main implication of these
studies is that a variety of different methods should be used during
the elicitation process. Nevertheless, certain types of approach will
obviously be more acceptable than others to particular individuals. For
example, some people may be happy to give direct assessments while
others will prefer the indirect approach.
Bunn and Thomas 7 argued that the use of devices such as probability
wheels might be most appropriate for ‘people who are generally
intolerant of ambiguity, those who prefer not to contemplate, or even
deny, the uncertainties of their judgment, or who do not accept that
psychological feelings can be quantified’. Indeed, Spetzler and Stäel
von Holstein 1 found that many people have difficulty in making direct
judgments, and even those who feel capable of assessing probabilities
in this way were often subsequently found to lack confidence in their
responses. Most subjects, they claim, prefer the probability wheel. For
this reason, they recommend that interviews should generally start
with assessments based on this device, while other methods should be
used at a later stage as consistency checks (by consistency checks we