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We investigated how people use base rates and sample size information when combining data to make overall probability judgments. Participants considered two samples from an animal population in order to estimate the probability of that animal being aggressive. Participants' judgments were influenced by subpopulation base rates when they were provided and linked to specific samples. When samples were not identified as coming from different subpopulations, judgments typically reflected sample size information. We conclude that 1) People can use base rates when combining samples to make an inference; 2) People can correctly use sampling information to determine when to use base rates, and 3) People are able to consider base rate and sample size information at the same time. Additionally, we found that individuals' numeracy correlates with the extent to which base rate and sample size information is used.

An experiment is reported in which S s are asked to compare pairs of binomial samples and judge which, if either, gives better evidence of asymmetry in their underlying population. The results indicate that whilst all S s are influenced by sample proportion only about one third give appropriate weighting to differences in sample size. However, according to the theory of ‘representativeness’ postulated by Kahneman and Tversky (1972) sample size should have been ignored altogether. A second experiment confirmed the hypothesis that their results reflected the use of over-complex problem presentation. When one of their problems was systematically simplified, a significant majority of S s showed insight into the role of sample size.