Futures Win Totals O/U - Win Inflation

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Futures Win Inflation
TL;DR: In the MLB Futures Market Win Total O/Us, the estimated hold you’re paying on the over will likely approach 8 percent while the estimated hold on the under can be close to zero. With a little line shopping between books, you can find some unders with +EV with little effort.
A new MLB season is starting and you want to bet the over/under on win totals for your favorite team, the Yankees. After all, they picked up Gerrit Cole, and their offense is as potent as ever.
You see your sportsbook is offering Yankees win totals at 101.5 -110/-110. You figure paying an effective hold of 4.5% (on -110 both ways) is not a bad way to bet your favorite team to have a good season.
Win Inflation
Unfortunately, sportsbooks are well aware of our homer biases and shade the win totals upwards, something we’ll call win inflation.
In the MLB we usually expect a total of 2,430 regular season games to be played (30 x 162 x ½). If 2,430 games are played, there are a total of 2,430 wins available for all 30 teams to share.
2020 Season - Win Inflation Across Books
We reviewed the season win totals markets of four sportsbooks. They all have win inflation, averaging 7 excess wins and ranging from one win to 13 wins.
Win Inflation
Odds Shading
By looking at the table above, you may say “Hey, DraftKings doesn’t incorporate much win inflation. They are the best place to place over wagers at.”
Nope. Books shade their lines against the over:
Vig Inflation
In addition to the win inflation, the sportsbooks are making you pay more to bet the overs.
The Cost of Win Inflation and Estimated Vig
We already know that the average vig is 4.7% on this market. However, with the odds shading the vig isn’t split equally between the over bettors and the under bettors. Over bettors are clearly paying more. For an event that *should* be a 50/50 wager, a bettor laying -113.5 is paying 5.9% in vig and a bettor laying -107.5 is paying 3.5% in vig:
Vig on O/Us
However, we still need to figure out the cost of the win inflation.
Let’s assume that an average team has a win expectation of 81 wins and the probability of them landing exactly on 81 wins is 4.5%. (We’ll save you the math, but we’re comfortable with this number based on some prior analysis of push probabilities.)
Due to win inflation, however, you are only offered Over 81.5 at -110 instead of Over 81 at -110 and your range of outcomes changes:
Expected Hold
That hook increased the expected hold from 4.34% to 8.84% (alternatively if you bet the Under at -110 that hook would reduce the expected hold from -4.34% to -0.25%). On average, let’s say that hook cost 4.3%. If every win total had ½ of a win added to it as win inflation (15 wins total for the entire league), we could estimate that the expected hold on overs would be about 8.8% and the hold on unders would be near zero.
However, win inflation was only 7 wins on average rather than 15 wins, so only 2.0% of additional vig was “charged” to the overs and “returned” to the unders. We made the adjustments for win inflation below.
Adjusted Vig
The estimated hold you’re paying on the over will likely approach 8 percent while the estimated hold on under can be close to zero. With a little line shopping between books, you could probably find some unders with +EV without too much effort.
Historical Results
Historical results confirm our analysis, with unders winning 52% over the last 5 years while overs only cashing 48% of the time.
Historicals
 

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