A verbal-numerical probability scale

We will present a verbal-numerical scale for eliciting probability assessments,
and report studies we undertook to test this scale. A (numerical) probability scale
is the best-known direct method to elicit probability judgments: presenting a
horizontal or vertical scale and asking judges to mark a position on it. The scale
is easy to understand and use; its drawback is that it gives judges few anchors to go by.

Two indirect methods that are often used are gambles and probability wheels.
A probability assessment is inferred from judges' choice behaviour in a controlled
situation, which supposedly gives more correct assessments. A major drawback of
these indirect methods is that they are difficult to learn and very time-consuming in use.

Research has shown that it does not really make much difference in the probability
assessments which elicitation method is used. For reasons of efficiency and
user-friendliness, a scale would seem best. But we think it may be improved
upon, because people are not always very comfortable giving their judgements
numerically. We therefore took the standard probability scale as a point of departure,
and refined it to include verbal expressions of probability.