Cognitive processes in judging cumulative risk over different periods of time

Abstract In a series of experiments subjects estimated the risk of dying during a year for different hypothetical persons. The risk to die varied across persons and was always presented as three different risks during three time periods (e.g., for person x: first period 8 weeks with risk 13 deaths per thousand persons and year; second period 20 weeks with risk 1.5 deaths per thousand persons and year; third period 24 weeks with risk 6.0 deaths per thousand persons and year). The results indicated that when the risk level information correlated higher with statistical risk than the exposure times, the judgment heuristics applied were quite efficient. However, when the exposure times explained more of the variation in statistical risk than the risk level variation the heuristics no longer worked so well. When the statistical risk was held constant the judgments were very highly correlated with the risk during the shortest time period having the greatest variation in risk level and greatest mean. Retrospective verbal reports indicated a number of different judgment heuristics. A common characteristic of these was a two-stage sequential information processing starting with a search for an anchor on the judgment variable of risk. In the second stage the anchor was adjusted in accordance with the remaining information considered important. An initial search of the longest exposure time for using the corresponding risk as the initial anchor was the most common strategy. After this, the remaining two time periods and their risks were considered when adjusting the anchor to obtain the final judgment. Analyses of the results in terms of the weights given the risk levels indicated proportionality with exposure time for the risk during the longest time period. To summarize, man seems to be a good intuitive statistician in the task studied here if the exposure times of different risks do not explain more of the statistical variance than the risk levels themselves.

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