  We present an extensive statistical analysis of the results of all sports
competitions in five major sports leagues in England and the United States. We
characterize the parity among teams by the variance in the winning fraction
from season-end standings data and quantify the predictability of games by the
frequency of upsets from game results data. We introduce a novel mathematical
model in which the underdog team wins with a fixed upset probability. This
model quantitatively relates the parity among teams with the predictability of
the games, and it can be used to estimate the upset frequency from standings
data.
