Typically there are 4-5 oddsmakers assigned to make lines for each of the major sports (college and pro football and basketball; MLB, NHL, boxing and golf). Each oddmakers bring unique opinions, strengths and weaknesses into the process.
Commonly, people are under the misconception that point spreads represent the oddmakers prediction of how many points the favorite will win the game by. This is not the case - their intent is not to evenly split the ATS result between teams. But rather their goal is to attract equal betting on both team sides. In other words, they want to create a line that is attractive to people betting one way and also people betting the other way (known as dividing the action).
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Dividing the action means the sportsbook is guaranteed a profit on the game because of the fee charged to the better (called juice).
Opening line is the first line created by the oddsmakers which is the sent out to all the sportsbooks. There is of course an entire method to the madness on how the opening line is created. It starts with each oddsmaker creating a line on each game that is based on their personal approach. This usually includes having current power ratings on each of the teams.
Power ratings are the oddmakers value of each team and they are used as a guide to calculate the preliminary point spreads on upcoming games. The power ratings are adjusted after each game a team plays. Non game factor examples that require adjustments to the team's power ratings are key player injuries and key player trades.
When a games power rating pointspread is figured, the oddsmaker will make changes to that line after considering each teams most recent games and previous games played against the opponent. Adjustments are also made after reading each teams local newspaper to get a feel of what the players and coaches are thinking prior to going into the game.