Notes
From Tversky and Koehler, p. 553: "Stanford undergraduates (N= 196) estimated the percentage of U.S. married couples with a given number of children. Subjects were asked to write down the last digit of their telephone numbers and then to evaluate the percentage of couples having exactly that many children. They were promised that the 3 most accurate respondents would be awarded $ 10 each. As predicted, the total percentage attributed to the numbers 0 through 9 (when added across different groups of subjects) greatly exceeded 1. The total of the means assigned by each group was 1.99, and the total of the medians was 1.80. Thus, subadditivity was very much in evidence, even when the selection of focal hypothesis was hardly informative. Subjects overestimated the percentage of couples in all categories, except for childless couples, and the discrepancy between the estimated and the actual percentages was greatest for the modal couple with 2 children. Furthermore, the sum of the probabilities for 0, 1,2, and 3 children, each of which exceeded .25, was 1.45. The observed subadditivity, therefore, cannot be explained merely by a tendency to overestimate very small probabilities."