How Sabermetric Luck Affects Betting Markets
We are less than 10% through the MLB season, but hot streaks and disappointing starts have already begun to change the perceptions of many teams around the league. The Brewers’ early nine-game winning streak turned some heads, while the Diamondbacks 4-14 start already has manager Kirk Gibson on the hot seat. Although some people will scream “small sample size,” take a look at how futures odds have changed for these two teams:
As you can see, the odds have swung quite a bit in a relatively short amount of time. This is evidence that the perception of bettors has changed regarding these teams. Not only does the perception of a team affect their odds to win the championship, but also their odds to win each game on a daily basis. Below is a chart that shows how the betting percentages for the two teams have fluctuated throughout this young season:
The Diamondbacks saw a recent uptick when the Mets came into town, but Arizona was promptly swept. You can expect very little public support for the Diamondbacks over the weekend as they travel to L.A. to take on the vaunted Dodgers. Meanwhile, you can clearly see public support increasing for the Brewers as bettors have jumped on the teams’ bandwagon following their hot start.
The question that we should be asking is whether or not this perception is justified. Are the Diamondbacks worse than we originally projected? Are the Brewers benefiting from some lucky bounces? We now have relevant data to help answer these queries.
ESPN’s Mark Simon provided this great chart which shows the relationship between a pitching staff’s number of hard hit balls against the opponent’s BABIP (batting average for balls in play).
Using this information we can determine a line of best fit and easily see which teams are getting “lucky” or “unlucky” on balls put in play. The five unluckiest teams based on this metric are the Indians, Nationals, Diamondbacks, Marlins, and Phillies, respectively. You would expect these teams to be suffering from opponents getting bloop hits or finding holes more often than mathematics indicate that they should. Over the course of a 162-game season, we expect the BABIP to regress towards the mean assuming the frequency of hard-hit balls remains consistent.
Another factor that must be considered is team defense. If a team isn’t particularly adept at getting to hard-hit balls, then the trend will not correct itself to the same magnitude of a team with a good (or even average) defense. There isn’t nearly enough data to calculate a teams’ zone rating but it is interesting the four of the five “unlucky” teams above are among the league leaders in team errors.
Getting back to the two teams of note, we would expect Arizona’s bad luck to regress towards the mean and start to benefit from balls being hit at fielders more often as the season progresses. Meanwhile, the Brewers’ pitching staff has been among the best in the league at limiting hard-hit balls by their opponents. Thus, their opponents’ BABIP is below league-average but is in line with what we would expect.