So after going through the 2007 data looking for the impact of the length of road trips it is pretty clear that the length of home stands and road trips don't account for the .540 winning percentage for home teams.
There are a number of potential factors driving the home filed advantage. Some are easier to quantify than others. Some of the first things to come to mind are:
- Advantage of batting last in the ninth so you know what you have to do strategy-wise to win the game. For example, if the game is tied going into the ninth then the road team wants to score at least one run but they can't assume that one run is enough. At the same time the home team goes into the bottom of the ninth of a tied game knowing that playing for one run wins the game.
- Fielding advantages of playing in the home park. This would be interesting to break down and I think that it is a significant factor. However, like most fielding questions, it would be challenging to measure this. I think that the differences in outfield fence configurations, wind patterns and the way turf effects bounces have a real impact on defensive performance between home and road teams.
- Hitters may have somewhat of an advantage batting where they have a familiar hitting background.
- The home crowd can provide that boost of support to provide confidence in the home team.
While all of these are real factors there is one area where there are some simple measurements to look at. Pitching. The chart below shows some core pitching stats from 2007.
| IP | K | BB | K/9 | BB/9 | K/BB | ERA |
| Home | 744 | 563 | 265 | 6.8 | 3.2 | 2.1 | 4.29 |
| Road | 703 | 510 | 271 | 6.5 | 3.5 | 1.9 | 4.65 |
The most obvious is ERA where the home pitchers have an ERA that is 8% lower than road pitchers. The problem here is that this stat is a cumulation of the efforts of the teams. Side note, the expected pyth winning percentage of the home team based on the ERA split above is .533.
The two things that a pitcher has the most control over are strikeouts and walks. Home pitchers have a 4.3% higher K/9 rate than road pitchers, an 8.2% lower BB/9 rate and a 12.9% higher K/BB ratio. So why are there these differences?
As noted earlier, there could be some advantage for home hitters being familiar with the batting background. This could help them judge balls and strikes a bit better at home. However, I think that there is a more important factor, the pitcher's mound.
Every grounds crew shapes their mound a little differently meaning that no two are exactly alike. It doesn't take much to throw a pitcher's mechanics off enough to make a few strikes turn into balls. The fact that the home/road walk rates appear to be impacted more than the strikeout rates supports the idea that there is a home-mound advantage for pitchers. If the mound is a significant factor in a pitcher's control then you would expect to see walks jump up.
I would be intersted in doing a deeper analysis on the expected impact of just the walk and strikeout rate differences on run production. I have a feeling that these factors make up a good chunk of the home/road run production splits and therefore a significant portion of the home field advantage.
Thoughts?