This study showed that accuracy of the estimated relationship between a fictitious symptom and a disease depends on the interaction between the frequency of judgment and the last trial type. This effect appeared both in positive and zero contingencies (Experiment 1), and judgments were less accurate as frequency increased (Experiment 2). The effect can be explained neither by interference of previous judgments or memory demands (Experiment 3), nor by the perceptual characteristics of the stimuli (Experiments 4 and S), and instructions intended to alter processing strategies do not produce any reliable effect. The interaction between frequency and trial type on covariation judgment is not predicted by any model (either statistical or associative) currently used to explain performance in covariation detection. The authors propose a belief-revision model to explain this effect as an important response mode variable on covariation learning.Animals and humans behave adaptively because they are able to detect the relationship between events (Alloy & Tabachnik, 1984;Killeen, 1981;Mackintosh, 1977). Knowledge on covariation or correlation is very important to the organisms' cognitive functioning (Kareev, 1995) because it is needed in order to carry out such tasks as classical conditioning, category learning, or causal induction (Kareev, 1995;Waldmann & Holyoak, 1992). Therefore, it is important to understand the conditions and the mechanisms underlying this ability. A lot of data exists on these topics in the animal conditioning literature, and some authors have argued that human contingency judgments are useful for these purposes (Shanks, 1987). In fact, recent reviews (Allan, 1993;Shanks, 1993;Young, 1995) have stressed the similarity between human contingency judgments and the animal conditioning data. These studies usually use two dichotomous variables; a cue (S) and an outcome (O), with present or absent conditions possible for each. These conditions produce four types of trials: (a) the signal and the outcome are presented together; (b) S but not O occurs; (c) only O is present; and (d) neither S nor O is shown.Human contingency judgments are affected by conditions such as contiguity (Reed, 1992;Shanks, Pearson, & Dickinson, 1989), outcome value (Chatlosh, Neunaber, & Wasserman, 1985), cue competition (blocking [Chapman & Rob-