In sports betting, an over/under bet refers to a wager in which the sports book predicts a number for a statistic in the game in question, and the bettors have to wager whether the actual result will exceed that prediction or end up being lower. In most cases, over under wagers require bettors to guess the combined score of both teams participating in the match.
In Super Bowl XXXIX, virtually every Las Vegas casino set the over/under for the score of the game at 46. Consequently, each bettor could wager that the combined score would either be lower or higher than this predetermined number. In the end, the combine score of that game was 45, which means that everyone who wagered an “under” won.
When setting up over/under bets, the goal of each sports book is to have an equal number of wagers on both sides. Theoretically speaking, this means that the operator could set the value at zero and then readjust based on the number of incoming wagers and in-house handicapping efforts.
In practice, the initial value is most often simply based on statistical research and anecdotal information, such as media reports or the injuries of key players.