Sports betting lines are created by oddsmakers, who analyze a team's or player's past performance as well as such variables as the weather, player injuries, the home field advantage, and anything else that might affect the outcome of the game. Statistics, and any patterns or trends the odds-makers discern in the statistics, form the basis for a betting line. For example, if Team A is strong on defense and Team B has a strong offense, an oddsmaker will look at how effective Team A has been in containing other teams with strong offenses and how Team A has performed in previous contests against Team B. Virtually every aspect of a game, from location to time to mental distractions, is used to predict how a given team or player will perform.

Some Las Vegas sportsbooks have in-house oddsmakers, but most use the services of a group called Las Vegas Sports Consultants to establish their odds and betting lines. LVSC also provides this service to the Oregon State Lottery — the only other state in the United States that permits and regulates sports betting — and to licensed European bookmakers. According to some estimates, LVCS sets the initial betting lines and odds for up to 90 percent of the licensed sportsbooks in Nevada.

Betting lines on sports are usually adjusted many times between the posting of the initial line and the time the game or event begins. There are two main reasons for this. First, injuries, weather, and other factors can change the statistical outlook of a game, so oddsmakers will adjust their lines to reflect those changes. Second, virtually no sportsbook wants to risk its own money on any given sporting event. Sportsbooks make their money from the commission — often called the “juice” or the “vigorish” — they charge on bets placed, so their ideal situation is to have approximately the same number of bettors on either side of a line. That way, the money from the losing bettors goes to pay the winning bettors, and the sportsbook still gets its juice.

In general, your betting line is locked in at the time you place your wager. Sometimes you'll get better odds by placing your bet early; other times, it might be to your advantage to wait. This is where your knowledge of both the sport involved and the way sportsbooks work comes in. If too many people bet on the favorite, the sportsbook likely will adjust the betting line to encourage more people to bet on the underdog. When this happens, the underdog often gets better odds than it deserves from a strictly statistical point of view, so it might be worthwhile to place your bet closer to game time. On the other hand, if you want to bet on the favorite, you might be better off to get your bet in early to give yourself the best odds.