Describes 3 experiments in which housewives (N = 142) estimated the probability (p) of the presence of a disease which had been indicated by diagnostic equipment. Although data given to Ss indicated p .5, Ss consistently estimated p .8. Successive trials altered the order of presentation of data and progressively reduced the data given. However, Ss always gave closely similar p values, accompanied by high confidence ratings. 2 hypotheses are examined to account for these findings. A 3rd experiment suggests the conclusion that the most important factor is that Ss import a rigid prior probability from their previous experience and ignore numerical data. (PsycINFO Database Record (c) 2006 APA, all rights reserved)