David Cameron at Fangraphs has a fascinating piece on home / road splits for pitchers on batting average on balls in play (BABIP). This seemed to fit in with some discussion we had here on Wandy Rodriguez's home / road splits.
First, some background on the Wandy discussion. Several commenters feel that Wandy is not a No. 2 pitcher because he can't pitch well on the road. I surmised that people are still thinking about Wandy's crazy H / R split in 2007 (2.94 vs. 6.37), and suggested his 2008 H / R ERA split isn't that abnormal (2.99 vs. 4.33).
Second, David Cameron's work found that a consistent and persistent lower BABIP exists on pitching at home versus on the road. This is fascinating, in part, because DIPS theory tells us that pitchers have very little control over BABIP, because the stat excludes events like HRs, foul outs, K's, and walks. Since Cameron says it better than I can summarize it:
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.
The H / R split in NL pitchers' BABIP for the 2008 season is 9 points: .296 (H) and .305 (R). BABIP is also higher for NL starting pitchers than relievers: .301 vs. .298.
The 2008 H / R split for Astros pitchers in 2008 was considerably smaller than the NL average: .294 (H) vs. .298 (R). By the way, Astros' BABIP split between starters and relievers was much larger: .306 (starter) vs. .276 (reliever). Make of that what you will.
Now, on to Wandy. In 2008, Wandy had a BABIP split as follows: .303 (Home) vs. .317 (road). The 11 point split isn't too much higher than the NL average of 9 points, but it is quite a bit higher than Astros' pitcher splits as a whole. in 2007, Wandy's BABIP disadvantage on the road was much bigger: .272 (H) vs. .319 (R), or 41 points, which seems like it reflects a big luck factor. My conclusion is that Wandy's home / road "problem" isn't as much of a problem as people think. But, hey, I could be wrong.
By the way, Wandy's BABIP advantage over RHB was quite large in 2008: .345 (LHB) vs. .303 (RHB). LHPs have been shown to have more control over BABIP than other pitchers, but in this case the platoon advantage is the opposite of what we expect (i.e., LHP is tougher on LHB). Just to check, Wandy had a similar platoon advantage in BABIP over RHB in 2007 (.292 for RHB vs. .322 for LHB). Not surprisingly, Wandy's overall OPS-against figures for lefty / righty were in the same direction as BABIP in 07 and 08. What is Wandy doing which makes him so tough on RHB?
Back to Cameron's point that BABIP is consistently lower for pitchers at home as opposed to road....he offers the suggestion that home fielders are more familiar with the ballpark dimensions and infield grass. And, some commenters point out that players' defensive stats tend to be better at home than on the road. It is worth noting that DER (defensive efficiency rate) is the reciprocal of BABIP, which naturally leads one to connect this effect to fielding. In the comments, other people suggest that road hitters may be worse, perhaps they are more fatigued from travel. For that matter, road pitchers may be more fatigued for the same reason, but according to DIPS the pitchers don't have much control over BABIP. Or do they?
Does Wandy's odd L / R BABIP split have anything to do with his higher than average H / R split? Heck, if I know. But it might be worth thinking up a possible hypothesis (which I don't have).