Weinstein (1980) asked Rutgers students to compare their own chances of 21 life events each (42 in all) with those of other students of their sex at their college, on a scale running from “100% less” to “5 times average”. For being fired from a job, the average answer was 31.6% below average, and for every student who rated their chances above average, about seven and a half rated theirs below. One student can truly be below average, but across a whole college the differences must average out to zero.
Weinstein (1980). Unrealistic optimism about future life events. Journal of Personality and Social Psychology.