Effects of Prevalence and Test Diagnosticity upon Clinical Judgments of Probability

The process of revising diagnostic probabilities was studied to examine the relative influence of prior probability and test diagnosticity at various levels of clinical experience. The aims were to show changes with increasing seniority, to explore the effects of perceived seriousness of a disease, and to demonstrate systematic biases in handling probabilistic information. To these ends, 4 case vignettes were presented to 169 medical students in the three final years of the medical course and 25 residents. The data presented included the prior probabilities of two diseases and the true positive fraction of a single clinical manifestation of the rarer disease.Results showed a slightly increasing reliance on prior probability with increasing experience. The less experienced seemed most influenced by test results and by perceived seriousness of the disease. In some vignettes judgment seemed to depend on representativeness. In others, the most plausible explanation of the diagnostic choice would have been availability. Marked case-to-case variation was noted for individuals and there was a general lack of systematic biases. Revision of diagnostic opinion often depended on preconceived notions, and prior probabilities tended to be ignored. These are clear indications for teaching the basics of decision theory to medical students and early post-graduates.