Ever since Voros McCracken’s DIPS theory came to light in 1999, people have begun to look at a pitcher’s batting average on balls in play, or BABIP. As Voros noted, variations in BABIP from league average regress heavily to the mean in future years, and it’s value as a predictive measure is quite low. This insight helped paved the way for things like FIP and evaluating pitchers by the outcomes they can control and a movement away from metrics such as ERA.
As more research was done, though, it was found that BABIP isn’t entirely random. Knuckleballers have significantly lower BABIP than a traditional pitcher. Left-handers tend to have some minor BABIP advantage, as do flyball pitchers (though what they save in BA they give back in SLG). However, when looking through the major league splits pages on Baseball Reference, I noticed one other type of pitcher that has a significant BABIP advantage – the home team pitcher.
Here’s a chart to illustrate what I’m talking about.
In every year from 1995 to 2008 (and probably before – I didn’t bother going back any further once I found this obvious of a trend), the batting average of balls in play allowed by the home team’s pitchers was lower than the road team’s pitchers. The two lines generally move together, so when league BABIP is up or down, it’s up or down for both home and road in proportional amounts. But the home line never crosses the road line. It gets close in 2004, when the gap is just two points, but then diverges back to the more normal five to 10 point spread.
Over that 14 year period, home team BABIP allowed is .295, while road team BABIP allowed is .302. We’re talking millions of plate appearances here, so a seven point spread is certainly significant. It’s essentially impossible for this to happen randomly. There is something inherent to being the home team that allows you to reduce the amount of hits you allow on balls in play. This is, for lack of a batter term, a home field advantage.
What could be causing this spread in BABIP between home and road pitchers? Isolating a single factor is going to be next to impossible, and in reality, it probably isn’t a single factor. Outfielders learn how to read the ball off the bat in a specific lighting based on repetitive experience. Infielders learn how the grass makes a ball spin at different speeds. Pitchers figure out where the ball carries and where it doesn’t and pitch away from the areas that can hurt them the most. Hitters pick up the ball coming out of the background quicker. GMs acquire players who fit the quirks of their specific ballpark. It could be any of these, none of these, or all of these.
But we know this – there’s a distinct advantage in being the home team in turning balls in play into outs. If a pitcher gets an inordinate amount of home starts, we shouldn’t be surprised if he beats his career BABIP.