Superforecasting: The Art and Science of Prediction

by Philip E. Tetlock

The answer lies in the tournament data. Barbara Mellers has shown that granularity predicts accuracy: the average forecaster who sticks with the tens—20%, 30%, 40%—is less accurate than the finer-grained forecaster who uses fives—20%, 25%, 30%—and still less accurate than the even finer-grained forecaster who uses ones—20%, 21%, 22%. As a further test, she rounded forecasts to make them less granular, so a forecast at the greatest granularity possible in the tournament, single percentage points, would be rounded to the nearest five, and then the nearest ten. This way, all of the forecasts were made one level less granular. She then recalculated Brier scores and discovered that superforecasters lost accuracy in response to even the smallest-scale rounding, to the nearest 0.05, whereas regular forecasters lost little even from rounding four times as large, to the nearest 0.2.22 Brian Labatte’s granularity isn’t bafflegab. It is precision—and a key reason why he is a superforecaster.