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Why do Betting Lines Change?

September 30, 2010 by  
Filed under General Information

We have already seen in previous articles like How do Sportsbooks Make Money on the Moneyline, that sportsbooks get their profit by taking an equal amount of bets on both teams in any given game. The income from the losing bets offsets the payouts from the winning bets, and the “vigorish” or “juice” is the sportsbooks profit. And we have also seen that (-110) is the standard “vig”, although not the only one. Point spreads and moneylines are set carefully with the hope that they will entice bettors to wager on both teams equally. So why would a sportsbook change these carefully calculated numbers after they have been posted for the public to wager on?

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The reason is that these betting lines are only the oddsmakers best guess about what kinds of odds will result in the equal distribution of wagers on both sides of an event. And while oddsmakers are very good at making these guesses, nobody can perfectly predict how the public will respond. Let’s say that the betting lines come out for an NFL game between the Washington Redskins and the Dallas Cowboys, and the Redskins are +3 in the point spread. Let’s say further that for some reason a lot of people believe that the Cowboys will cover the spread easily, and in fact will win the game by 6 or 7 points, and they start betting heavily on Dallas.

Right away this causes an imbalance in the wagering patterns with a lot more money being placed on Dallas than on Washington, and that, as we know is contrary to the outcome that the sportsbooks are trying to achieve. This isn’t something that the sportsbooks like, but it certainly doesn’t come as any surprise to them. And neither is it any great cause for alarm either. It is simply an indicator that the betting lines need to be adjusted to persuade more bettors to lay money on Washington.

To do this, the sportsbooks will start increasing the point spread from 3 points to 4 or 5 or 6 points or whatever is needed to prompt the public to start betting on the Redskins. These adjustments are made very quickly by the sportsbooks and they will not stop moving the betting line until they get the desired result. Once equilibrium is established and wagers on both teams are as equal as they can be, the betting line stops moving. It will probably remain at that value which resulted in equilibrium unless some event occurs that causes the balance to be upset, in which case another adjustment is made.

When betting on NFL or NCAA football, several things can happen to make this adjustment necessary. A lot of this has to do with the fact that football games are held just once a week. Injuries may occur while players are practicing and this will affect the betting lines that were created earlier in the week. Weather forecasts change as time goes by and this also has an effect. The prospect of a sloppy, rainy day on a muddy field for example encourages betting on the team that can run the ball more effectively since bad weather hinders the crisp routes and split second timing needed for a successful passing attack.

There is at least one other major reason that the line is subject to change for football games and it again has to do with the weekly schedule. The line may change markedly on game day in response to high betting volume by last minute customers. On game day, all the information needed to evaluate a team’s chances of winning are in place. Weather predictions are at their most accurate. Injured or questionable players are as healthy as they are going to get and coaches will usually give an indication of whether these players will see action.

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In some cases other football games are taking place (or have already been played) on the same day that affect the importance of other games, especially near the end of the season. A team that has been knocked out of contention for the playoffs by the results of another game earlier in the day is not likely to be as motivated to play as an opponent who is fighting for its post season life and must win to stay in the hunt.

The result of all these elements is that betting on one team may become very heavy in the time period just prior to the game. A sportsbook will make whatever changes are required to the betting line to offset this massive influx of bets by making bets on the opposing club very attractive. They will continue to adjust to the wagering conditions right up until kickoff.