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January 17, 1998
Picking Winners
One of the attractive features of spectator sports is the uncertainty of the outcome. Even when one team is overwhelmingly favored to win, the underdog may come through with a surprising victory.
Nonetheless, the ability to pick winners can be of considerable interest, especially to gamblers, who bet on the outcomes of games, and to the oddsmakers, who in effect determine the playing field for the bettors.
In the current issue of Chance, statistician Hal S. Stern of Iowa State University in Ames takes a look at what sort of simple information may be helpful for identifying winning teams, though not necessarily for making bets that beat the spread (or odds). "The question of primary interest is what proportion of game outcomes could be correctly predicted by an intelligent observer," Stern says.
Stern focuses mainly on U.S. professional sports, though his analysis can be easily applied to other sports, as long as the right sorts of data are available.
One simple prediction rule is to pick the team that is playing at home. "This rule ought not to predict very well, because it completely ignores the relative ability of the teams that are competing," Stern remarks.
Nonetheless, the evidence supports the existence of a home-field advantage (see table), especially in basketball. Moreover, the home-field advantage for college sports appears to be slightly larger than for professional sports, Stern says.
Additional analysis indicates that playing on one's home field rather than at a neutral site is worth about 3 points in football, 4.5 points in basketball, and 0.25 run in baseball.
Sport Home Team Win/Loss Scores Oddsmakers Pro football .58 .63 .65 .67 Pro basketball .66 .69 .70 .71 Pro baseball .53 .56 .56 .55 College football .62 .69 .73 .75 College basketball .68 .72 .72 .75 Oddsmakers employed by sports betting establishments make a living by forecasting the outcomes of games, though their goal is not so much to make accurate predictions as to set the odds (point spread). When they succeed, the proceeds from losing bets pay off the winning bets, with a small percentage going to the betting establishment.
Oddsmakers' predictions generally prove to be a superior guide for identifying winning teams. Inbaseball, however, the oddsmakers do only slightly better than the rule of always picking the home team. Oddsmakers do somewhat better at predicting basketball and football outcomes. College games tend to be more predictable than professional contests in the same sport.
It's also possible to apply simple statistical techniques (such as the method of least squares) to the win-loss records or margins of victory of the participating teams in previous games. In effect, the methods provide an estimate of the ability of each team.
In football, Stern's rudimentary statistical approach does nearly as well as the experts and considerably better than the strategy of always picking the home team, particularly when scores are used. A similar pattern occurs in the other sports.
"Baseball appears to be the most random sport," Stern concludes. "The best prediction approaches are just a bit better than using coin flips to predict."
In basketball and football, prediction accuracy can reach 75 percent, but that still leaves plenty of uncertainty. "One might argue that if things were any more predictable than that, it would be difficult to convince people to pay for the privilege of watching the games!" Stern notes.
References:
Stern, H.S. 1997. How accurately can sports outcomes be predicted? Chance 10(No. 4):19.
Comments are welcome. Please send messages to Ivars Peterson at ip@sciserv.org.
Ivars Peterson is the mathematics and physics writer and on-line editor at Science News. He is the author of The Mathematical Tourist, Islands of Truth, Newton’s Clock, Fatal Defect, and *The Jungles of Randomness: A Mathematical Safari. His current works in progress are an updated, 10th anniversary edition of The Mathematical Tourist (to be published in 1998 by W.H. Freeman) and Fragments of Infinity: A Kaleidoscope of Mathematics and Art (to be published in 1999 by Wiley).
*NOW AVAILABLE: The Jungles of Randomness: A Mathematical Safari by Ivars Peterson. New York: Wiley, 1997. ISBN 0-471-16449-6. $24.95 US.
copyright 1998 Science Service