Visual statistical decisions

To identify variables that underlie intuitive judgments about the sizes of groups of similar objects, we asked people to judge the relative heights of vertical bars briefly shown, two groups at a time, on a computer display. Randomly selected normal deviates determined individual bar height. Average differences in height and group sizes were also randomly varied. Twenty-eight participants judged 250 differences each, which were then submitted to multiple regression analysis and psychophysical inspection. The total number of bars sharpened discrimination, whereas variance dulled it. Critical ratio (CR), the forerunner to the modern t test, emerged as the most important predictor, little additional variance was explained by other factors. The difference in the number of bars was a reliable factor, favoring the greater number of bars. Confidence limits around thresholds, defined as CRs needed to say "possibly greater" surrounded 1.65; as a z value, this corresponded to a one-tailed probability of .05. Judgments about noisy stimuli thus seem to be based on a statistical process and to employ a probability criterion similar to that used in the formal statistical evaluation of experimental findings--namely, p < .05.