BABIP and Home Field Advantage

With several recent discussions (here and here and here and here) on home team advantage (HTA) – which began with Tobias Moskowitz’s and L. Jon Wertheim’s new book Scorecasting – I decided to see if I could find any reasonable causes for the advantage. I decided to look into areas that I thought home teams may have an advantage, namely errors (not much – about 2 wins league wide) and base running (some), but the number that caught my eye was the differences in batting average on balls in play from the home and away team. Here are the differences in BABIP for the home and away teams over the last few years:

As you can see, there is an average difference of about 0.007 BABIP difference between the home and visiting team. This difference is quite substantial when looking over an entire season, working out to about 230 hits per season for the entire league. Using an average of 0.54 runs created per non-home run hit (thanks Tommy), the run difference works out to be 123 runs or about 12 wins across the entire league. This effect accounts for about 17.5% of total home field advantage.

There are many potential causes for the difference – for instance, the away pitchers may be squeezed by the home plate umpire, leading to hitters counts and more hard hit balls. Another theory is that the home team’s defenders know the park better and can make more plays on balls hit into the field which lead to more outs. To answer these questions I used Retrosheet batted ball data from 2006 to 2009 (I have not yet updated my retrosheet data with 2010 info yet) to look for possible differences:

The main difference in the percentage of batted ball data is that the home team hits more line drives versus the away team, who tend to hit more fly balls. This will explain some of the overall difference since line drives are more likely to end up as hits. By adjusting the away team’s batted ball distributions to be the same as the home team’s distribution, the visitors BABIP raises to 0.299.

I am not sure of the exact cause of the difference, but here are some questions that could be explored: Do home pitchers feel comfortable throwing pitches that will end up as fly balls knowing that the ball will stay in the park? Could it be the same as the previous reasoning with the home town hitter knowing what they can’t hit out of the park and settle for line drives or ground balls instead? Could it be that the home hitters are able to take advantage of their home park’s dimensions, or are General Mangers just specifically compiling rosters to take advantage of how their parks play?

The rest of the difference in BABIP is shown by the differences in the rate that the batted balls become hits. BABIP for both pop ups and flyballs are the same between the home and away teams. The home teams have a nine point advantage with ground balls and a five point advantage with line drives.

The reasoning behind this difference can’t be easily explained. Is it the pitcher having to throw more pitches in the heart of the plate? Do the away infielders not know the infield well enough to predict all the possible bounces a ball can take? Do away fielders have a more difficult time seeing hard hit balls leaving the bat? Is there a home town bias in scoring different batted ball data?

I feel like I am just beginning to scratch the surface, but I am working on filling in the gaps. For one, I am looking into getting the home and away defensive splits, especially for the infielders vs. outfielders. I would like to see which fielders are being labeled as responsible for these extra hits (all infielders?, 70-30 with the outfielders?). Second, I am looking at taking the single month’s worth of Hit F/x data publicly available and checking to see if there are any differences in batted ball speed and distribution. While I set out to answer a few questions about home field advantage, I feel that I have raised just as many, if not more, questions. This is a subject that simply requires more research.