The purpose of this experiment was to examine the degree of fluctuation in "cognitive probability" acquired by Ss in observing the appearance of "one of two events" in series. A rotating-drum was so arranged that a white or red ball would come out before a subject when he turned the handle. He was asked to judge and tell how many white balls were to be contained among 100 balls in the rotating-drum. Therefore, in this situation, the S presumed that the order of appearance of balls was at random in each trial. But, in fact, sending out of each ball was controlled by E according to the experimental series set up in advance. 40 Ss, men and women of the 1st year of Keio University, were devided into 4 groups of 10. The series assigned to each group were as follows: [table] At first, the first series was presented to each S, and then second series followed. E recorded "the number of white balls" S told on each trial. The results were as follows: 1) "The number of white balls" judged by S in the trials on the first series was fewer than the mathematically expected number. 2) Degree of modification of the above-mentioned "number" through the trials on the second series was larger in Group I and III than in Group II and IV respectively, and was larger in Group III and IV than in Group I and II respectively. 3) In each group, both on the first and second trials, the degree of fluctuation of "the number of white balls" judged in each trial had a tendency to decrease as the trials went on. This tendency revealed that the degree of fluctuation was greater than mathematically expected.
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