It used to be that only sports fans watched the World Series. But in the Leverett House library a lecturer on social psychology is conducting an experiment on subjective probability--using the annual Series clash as back- ground for his experiment.

Fifteen times during yesterday's contest Elliot Aronson asked students watching the televised game to estimate the chances that the batter would hit safely, strike out, hit a home run, etc. The purpose: to determine the influence of situational factors on the viewer's answer.

Such factors, Aronson said, will distort the judgment of probability in one direction or another. With Mantle at bat in the bottom of the nineth, two outs, men on base, and the Yankees behind, the sports fan might rate the chances for a home run as high as 50-50. Ordinarily, however, the chances are only 1 in 12.

On an accompanying questionnaire the viewers indicate whether (and to what degree) they like and dislike the Yankees and the Reds. A final question, seemingly out-of-place, reads: "How many older brothers and sisters do you have?" But Aronson revealed that first-born children "react more strongly to anxiety-producing situations," such as a crucial play in the final game.

Knowledge Tested

Finally the questionnaire asks for factual responses--what was Roger Maris' batting average this season; how many times did Louis Aparicio steal this year; what was Warren Spahn's 1961 pitching record. These questions gave Aronson a clue to the viewer's knowledge of baseball--important, Aronson said, because the more one knows, the closer he will be to predicting the real probability.

But all is not work in the Soc Rel Department. "One motive behind this experiment," said Aronson, "is that I want to watch the Series."