Anchoring in the interpretation of probability expressions

Prior research has demonstrated that people's interpretations of nonnumerical probability expressions (e.g. slight chance, probably) are sensitive to contextual factors such as event base rate. The present study examines the contextual role that numerical anchors may play in interpretation of these expressions. Participants read a series of vignettes describing fictitious doctor-patient communications in which the doctor used a nonnumerical probability expression to describe the likelihood of an illness given certain symptoms. Participants interpreted this expression (i.e. made a percentage probability estimate of the illness likelihood) in the context of a numerical anchor which they were led to believe was arbitrarily chosen. Despite the ostensible arbitrariness of the numerical anchors, interpretations of the probability expressions were systematically influenced by anchor value. These results are consistent with previous demonstrations of the 'anchoring-and-adjustment' heuristic and offer further evidence of the context sensitivity of probability expressions.