The Science Behind Point Spreads

Fantasy The Science Behind Point Spreads

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Just who is this “Vegas” I’ve heard so much about?

The Super Bowl offers a host of wagering options.  It’s an opportunity for the uninitiated to get exposure to a wide variety of wagers.

Some like block pools or drawing names for the game’s MVP from a hat are simply attempts to randomly redistribute money without the need for skill.  But the bulk of football-watching America has an external-to-the-game interest involving either fantasy football or some form of wagering against the spread, over/under, money line, etc.

If you know how to deal an in-game line, have been targeting middles for a decade, or can calculate the arbitrage opportunity in a set of futures lines, then this piece isn’t for you.  But I want to address the questions I hear most often from people who either gamble casually or are just hearing about gambling.

First, some basics…

What is a point spread?

Because football teams are of varying skill levels, one would be selected more often if it was possible to simply wager on the winner without adjustment.  In order to split the dollars wagered, bookmakers set a point spread or “spread” which gives one team an advantage (and the other a disadvantage) with regard to the outcome for wagering purposes.  The following phrases are equivalent:


  • The line is Baltimore +4
  • The line is San Francisco -4
  • Baltimore is a 4-point underdog
  • San Francisco is a 4-point favorite

If the 49ers win by 5 or more points, the people who bet on the 49ers win.  If the 49ers win by 3 or less or lose outright by any score, the people who bet on the Ravens win.  If the 49ers win by exactly 4 points, all wagers “push” (tie) and no money changes hands.

Does that mean I can make such bet at even money?

Not exactly.  The bookmaker must make money, so he has to receive something in exchange for taking the bet.  For most US casinos and local bookies, the bettor (also known as the “punter”) must wager $11 to make $10 if he wins a bet with the spread.  If the bookie has 2 such wagers, 1 on each team, he’ll make $11 on the losing wager and lose $10 to the punter who has the game called correctly.  That $1 for every $21 changing hands is known as his vigorish, or simply “vig”.  On pro sports it’s a fairly-steep 4.76% (1/21) since the punter must win 11 out 21 wagers to break even.  That’s almost as bad as roulette (5.26%), and worse than some slot machines.

Why do point spreads come in half-point increments when the teams can only score a whole number of points?

With whole number point spreads there is a chance of a tie.  In that case, all wagers tie (“push”) and no money changes hands.  With half-point spreads, there is no chance of a tie.  The bookmaker would typically prefer a half-point spread if he can split the action to guarantee a profit, but often that can only be accomplished with a spread that is a whole number of points.

Do all bookies offer the same point spreads?

No, but in the cyber-connected world we live in, they are aware of competitor movements almost instantaneously.  There are several services available to the general public which will allow gamblers or other interested parties to look at point spreads offered by multiple online books to “shop” for the best bargain.  I like Oddschecker for my writing, because it gives lines from approximately 20 books and virtually every book will have all the games available.

I’m confused by point spreads and just want to bet on the winner, how do I do that? 

You can do that using something called the “moneyline”.  For the favorite, that means you’ll get less than $1 of winnings per dollar you wager.  For the underdog it means you’ll generally get more than $1 of winnings per dollar you wager.  The moneyline on the favorite is expressed in terms of how much you have to bet to win $100 and is followed by a “-“ sign.  So if you want to bet the 49ers to win the Super Bowl, the moneyline would look something like SF 161-.  So you’d have to bet $161 to win $100 if you’d like to bet SF to win the game without a point spread adjustment.  The underdog is expressed in terms of how much a $100 bet wins and followed with a “+” sign.  For the Ravens, that number would be something like 138+.  So you win $138 if you bet $100.  And you thought that would be less confusing.

Now that you know the basics, there are a few central tenets for gambling:

The line is dollar weighted, not count-of-idiots weighted. 

This is another way to say the smartest money wagers the largest amounts.  I don’t have data to support this claim, but in order for lines to be in balance there must be a majority of wagers on one side that is made up by larger wagers on the other side.  While many books seek to avoid having a wager on the game, that is effectively what they have with unbalanced action.  In that case, the book may attempt to “lay off” some of that risk on another book (possibly with unbalanced action in the other direction).  When the aggregate unbalance of action across the market becomes significant, there is no choice but to change the point spread to balance new action.

The book’s biggest fear is moving his line when a lot of money has already been wagered. 

Let’s do this by example.  Let’s say the Ravens are 2.5 point underdogs versus the Steelers in a game at Pittsburgh.  The early action is heavy on the Steelers, and the line moves to 3.5 (this would be an enormous jump, but we want a clear example).  The betting public then wagers heavily on the Ravens before the line then stabilizes at Pitt -3.  The game is then played and the Steelers win by exactly 3 points.  The heavy early action on the Steelers covers (wins) because they won by more than 2.5.  The heavy action late on the Ravens covers because they lost by less than 3.5.  Bookies refer to this as getting “middled”.

The entity setting the line doesn’t “disrespect” your team, the gamblers do. 

In a message board post I’ll often see a question like “why does Vegas hate us?”  The people setting the lines don’t care about the Ravens.  They don’t care about Ray Lewis, they don’t care what Joe Flacco should make next season, and they don’t care when Bernard Pollard says the NFL won’t be around in 30 years.  They only really care about how the betting market will price the game so they can closely approximate the spread when they first offer it and hone it before many dollars are wagered to something that will split action appropriately.  So when your team is a 9.5 point underdog, don’t blame “Vegas” or the bookies.  Blaming the gambling public might be appropriate, but…

Complaining about the line does no good and may cause you to miss an opportunity. 

When I hear people complaining about the point spread, I’m always reminded of a scene for the classic film The Good the Bad and the Ugly. 

A mispriced line is an opportunity, not an affront.  So when it’s time to bet…bet, don’t talk.  You’ll have time to talk when you’re collecting.

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Ken McKusick

About Ken McKusick

Known as “Filmstudy” from his handle on area message boards, Ken is a lifelong Baltimorean and rabid fan of Baltimore sports. He grew up within walking distance of Memorial Stadium and attended all but a handful of Orioles games from 1979 through 2001. He got his start in sports modeling with baseball in the mid 1980’s. He began writing about the Ravens in 2006 and maintains a library of video for every game the team has played. He’s a graduate of Syracuse with degrees in Broadcast Journalism and Math who recently retired from his actuarial career to pursue his passion as a football analyst full time. If you have math or modeling questions related to sports or gambling, Ken is always interested in hearing new problems or ideas. He can be reached by email at [email protected] or followed on Twitter @filmstudyravens. More from Ken McKusick


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