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Durham University



Publication details for Professor David Over

Evans, JSBT, Handley, SJ, Perham, N, Over, DE & Thompson, VA (2000). Frequency versus probability formats in statistical word problems. Cognition 77(3): 197-213.

Author(s) from Durham


Three experiments examined people's ability to incorporate base rate
information when judging posterior probabilities, Specifically, we
tested the (Cosmides, L., & Tooby, J. (1996), Are humans good intuitive
statisticians after all? Rethinking some conclusions from the
literature on judgement under uncertainty. Cognition, 58, 1-73)
conclusion that people's reasoning appears to follow Bayesian
principles when they are presented with information in a frequency
format, but not when information is presented as one case
probabilities. First, we found that frequency formats were not
generally associated with better performance than probability formats
unless they were presented in a manner which facilitated construction
of a set inclusion mental model. Second, we demonstrated that the use
of frequency information may promote biases in the weighting of
information. When participants are asked to express their judgements in
frequency rather than probability format, they were more likely to
produce the base rate as their answer, ignoring diagnostic evidence.
(C) 2000 Elsevier Science B.V. All rights reserved.

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