Eliciting expert beliefs in substantial practical applications Read before The Royal Statistical Society at ameeting on 'Elicitation' on Wednesday, april 16th, 1997, the President, Professor A. F. M. Smithin the Chair

The practical elicitation of expert beliefs is considered through two contrasting examples. The first example concerns elicitation of engineers' prior beliefs about various quantities relating to the future capital investment need of a water company. Prior beliefs needed to be elicited about very many quantities, but only in the form of prior means, variances and covariances. A computerized procedure was required that could be routinely used by engineers, unsupervised. The second example is a single, application-specific elicitation of the beliefs of hydrogeologists about properties of certain rocks. A full probability specification was ideally to be obtained from a two-day intensive, supervised, elicitation with several experts together. The two different approaches used for these problems are described and contrasted, but a common principle of trying to identify and elicit separately the various sources of expert uncertainty is identified