Perception of correlation reexamined

Almost all studies of adult notions of correlation between dichotomous variables show that people do not incorporate two conditional probabilities as they should according to normative definitions. However, these studies disagree considerably about what correlational notions people do have. This paper identifies three factors that contribute to the variability in research results. The first two factors were mentioned in the literature, and the evidence concerning them is summarized: (1) the way data are presented and (2) the instructions subjects receive. A third factor is suggested and studied; the type of variables between which correlation is judged may affect subjects' notion of correlation, Specifically, asymmetric, present/absent variables (e.g., symptom: present, absent) may strengthen the incorrect notion of correlation as the tendency of two events to coexist (e.g., presence of symptom and presence of disease) disregarding the complementary events. In three experiments, subjects were asked to choose among five interpretations of the sentence "A strong [or no] relationship exists between [two variables]," The above prediction was confirmed.