On combining diagnostic 'forecasts': Thoughts and some evidence

The combining of forecasts involves more than issues of statistical aggregation. This comment focuses on situations where people have to combine forecasts in the form of diagnostic opinions concerning different states of nature. Taking a descriptive or psychological viewpoint, it is argued that people act upon the information they obtain and engage in considerable interpretation and imagination. Three specific problems are discussed: (1) the level at which opinions are aggregated; (2) the effects of redundancy and credibility of different sources; and (3) the manner in which the structure of information from multiple sources can lead to different diagnostic interpretations. The discussions are illustrated with experimental data.