Subjective correlation and the regular-random numerosity illusion.

It has been shown previously that a regular pattern of dots appears more numerous than a random pattern with the same physical number. Birnbaum and Veit's theory of contrast with expectancy has been suggested as a possible interpretation of this illusion. In order to test this theory, three groups (N = 102) were given prior exposure to stimuli with either a positive, negative, or zero correlation between regularity and number. The predicted interaction between correlation and item arrangement was not found. The main effect of regularity was equally strong in all three groups.