Three experiments examined people's ability to incorporate base rate information whenjudging posterior probabilities. Speci®cally, we tested the (Cosmides, L., & Tooby, J. (1996).Are humans good intuitive statisticians after all? Rethinking some conclusions from theliterature on judgement under uncertainty. Cognition, 58, 1±73) conclusion that people'sreasoning appears to follow Bayesian principles when they are presented with informationin 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 thanprobability formats unless they were presented in a manner which facilitated construction of aset inclusion mental model. Second, we demonstrated that the use of frequency informationmay promote biases in the weighting of information. When participants are asked to expresstheir judgements in frequency rather than probability format, they were more likely to producethe base rate as their answer, ignoring diagnostic evidence.
The CAUSE Research Group is supported in part by a member initiative grant from the American Statistical Association’s Section on Statistics and Data Science Education