Proportionality and sample size as factors in intuitive statistical judgement

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.