Last week I wrote an article on home field advantage and what types of parks are conducive to particularly large or small home field advantages for a team. Its conclusion was that unusual and idiosyncratic parks give teams the biggest home field advantage. This week's column expands on the topic, tweaking the model a bit and studying additional effects of home and road performance of teams in various ballparks.

Last week I suggested that since unusual parks give the largest difference between home and road WPCT, that unusual parks were the most advantageous to teams overall. The assumption was that all teams play to their true skill level on the road, with the home park effect taking hold only when the teams were at home.

Several people challenged this assumption and hypothesized that perhaps an especially high difference between home and road performance may be due to an unusual home park giving teams a road disadvantage. This is a very difficult distinction to make, but here I'll try to do so statistically.

One way to look at this is to test for a correlation between overall winning percentage and the difference between home and road records. A positive correlation would indicate an overall positive effect for having a high home/road difference, while a negative correlation would indicate the road disadvantage that others have hypothesized about. However, the correlation (which we would expect to be very weak in any case) between home/road difference and overall winning percentage was not significant either way. The p-value of the correlation was .61 when using year-by-year data, and was .96 when using aggregated winning percentages for each park. So this is an inconclusive test - we don't see evidence of a high home/road difference really helping teams, but we don't see it hurting either.

Another approach is to take a look at players who moved from a regular park to an idiosyncratic park, or vice-versa. If the road disadvantage hypothesis is correct, then we would expect the raw road performance of players to decrease when playing for the team with the unusual park.

For this, I did a case study of two parks with extremely high home/road splits where this road disadvantage might be evident: Coors Field and the Astrodome. One of course, is an extreme hitters park, whereas the other is an extreme pitchers park. Both conferred a very large home field advantage to their teams.

Overall I found 131 cases of players moving either in or out of these two parks in adjacent years with significant playing time (250 PA's for hitters, 100 IP for pitchers). For both hitters and pitchers I looked at the difference in Road OPS when playing for either the Rockies or Astros and when they were playing for another team in an adjacent year.

Unfortunately, this method is fraught with variance. Players' statistics can change dramatically from year to year for many other reasons besides what park they are playing in, and this clouds the study. Additionally, bias could be introduced due to the effects of coaching staffs and other factors. Due to this and a relatively few number of cases, it is difficult to detect if such a road disadvantage is occurring. The results of the study can be seen below.

Both Rockies hitters and pitchers tended to have better road statistics in years when they were not playing for Colorado. Astros pitchers also had this same result, but Astros hitters performed better on the road when they were playing for Houston than for another team. When taking all 131 players together, we see an overall decrease in road performance when playing for the team with an unusual park of 7 OPS points. However, the standard error of this estimate is 10 points of OPS, meaning that we are far from being able to make any conclusions on whether an odd home park really causes players to perform more poorly on the road.

In lieu of concrete statistical results, a discussion might be useful. The original findings were that teams with unusual home parks tend to have larger home/road splits. A good reason for this may be that visitors, not familiar with the park, may not be able to deal with the park's quirks as well as the home players can, since they have had more practice.

Certainly it would seem that learning one park's difficult oddities wouldn't cause a player to forget how to play in normal parks. A sailor who learns on rough seas can sail in calm seas as well. It seems doubtful that an outfielder who learns the Wrigley wind and ivy should suffer any disadvantage when playing elsewhere - after all, the conditions are easier and he still gets plenty of practice playing 81 games per year on the road. Similarly, will a player who plays in a dome forget what it's like to play outside even though he does so for nearly half the season on the road? To me it seems as though if unfamiliarity is the main reason for high home/road splits, then a player's road performance would be consistent, since all players are familiar with playing on the road (in a variety of parks) half the year.

However, it does seem as though a player playing in an extremely unusual ballpark could develop bad habits that would carry over to his road games, giving him a road disadvantage. For instance, playing one's home games in the LA Coliseum could cause players to get into the habit of popping up balls down the line for cheap homers - a habit that would cause him great harm in most normal parks. How much players can control their habits depending on their surroundings is unknown, but this could be a reason why an odd home park could cause a road disadvantage. Whether the road disadvantage would outweigh the home advantage in this case is a matter of debate.

After reviewing all of the evidence and arguments here, I'm still inclined to say that teams with quirky home parks are helped overall by their park and I would highly doubt that teams are actually hurt overall by having a quirky park. I will say this with one caveat however, and that is that teams with an odd park could have trouble attracting top talent (such as pitchers to Coors Field or hitters to the Astrodome) and in the era of free agency, this could be a very big disadvantage indeed. However, that is a conversation for another day.

Update to Last Week's Study

Last week, a thoughtful commenter made a great point that perhaps Coors Field carried too much weight in my study, considering the fact that the magnitude of its home field advantage is an outlier and its altitude makes it a unique park, not replicable elsewhere. Looking at a few regression diagnostics, indeed Coors Field had a large impact on the findings in the model - and while its inclusion is defensible, it's probably preferable to have its influence lessened by reducing its weight considerably. After doing so, the basic results are the same - unusual parks are of the most advantage - however, I'd like to share a few additional findings that this change produced.

The findings were, in order of importance:

1) Parks which were subjectively considered "quirky" had a greater home field advantage. This was still the most important predictor of home field advantage. P-value <.001

2) Parks which produced a lot of doubles still produced a high home field advantage. P-value=.002

3) Domed stadiums produced a higher home field advantage. This new finding was partially due to a reconfiguring of the "dome" variable, as well as the reduction of influence of Coors. P-value=.016.

4) Pitchers parks provide a greater home field advantage, though being a hitters park is not a disadvantage. In fact, playing in a hitters park is better than playing in a neutral park. The following graph shows the relationship between park factor and home park advantage. This is a change from last week's findings, which showed that hitters and pitchers parks were both equally superior to neutral parks - however, without the over-influence of Coors, we find that pitchers parks clearly provide a higher advantage.

5) Strikeouts and triples, which were marginally significant before, are now not at all significant.

Additionally, I wanted to give a complete list of ballparks and their predicted home field advantages for readers to use as a reference. It's also particularly interesting for relatively new parks, where the predicted value may be more accurate than the actual empirical home field advantage since this is highly variable over only a few seasons.

Of parks built in the last 10 years, Minute Maid Park and AT&T Park should continue their excellent home field advantage, while Petco Park, so far not giving the Padres much advantage, should improve to be a very advantageous parks. On the other side, Citizens Bank Park, which has so far provided a very poor advantage, should continue to do so (though not quite as bad as its been), and New Busch and Nationals Stadium should see their home field advantage decline from what it has been during each stadium's first few years. When we check back in 20 years or so, we'll see if these assessments have been correct.