Minor Matters: Home Field Advantage in the Upper Minor Leagues

The AMLU umpires are still on strike and I’m still trying to find out what effect, if any, the replacement umpires have on the game of minor league baseball. One of the more interesting explanations I heard in response to my earlier article about the effects of replacement umpires suggested that home teams enjoy an advantage because they are getting to know the local umpires.

In lieu of their regular traveling umpire crews, the minor leagues are hiring local umpires from college and high school leagues. Although baseball officials will not release any information about the replacement umpires, it is reasonable to assume that many replacement umpires are working most or all of their games in one city. If this is true, the home team might have an advantage because they will develop an understanding of the local umpires’ strike zone judgments as the season progresses.

I’m going to explore this possibility by looking at trends in strikeout-to-walk ratio differences at home and on the road in several different leagues. If hometown players have an advantage because they are getting to know the local umpires, then we should see increasing differences in weeks five through eight of the 2006 season.

International League

Trends in the International league support this idea:

The last week is an exception, but the differences in home and away results generally seem to be getting more extreme.

We will eventually need new park factors to understand prospects’ performance in this context. If a pitching prospect suddenly improves their walk rate, for example, it will be useful to know what kind of home field advantage might have influenced that walk rate. I will wait until mid-season to formally develop new park factors, but for now I will let you know what teams are enjoying the most extreme home field advantages at this point in the season. In the International League, Syracuse, Ottawa, Columbus, Charlotte, and Buffalo hitters are all striking out less and walking more at home than they are on the road. The reverse is true of pitchers for those teams.

Pacific Coast League

The home field advantage seems very consistent in the Pacific Coast League, but the results do not support the “getting to know the locals” hypothesis. The home field advantage has been decreasing over time, and the splits for the month of May are just slightly more extreme than the splits from one year ago. The overall difference is still statistically significant but it looks like things might be changing.

Four locations stand out with some extreme splits that favor the home team: Round Rock, Oklahoma, Memphis, and Colorado Springs. The Sky Sox hitters are walking twice as often at home than on the road.

Visiting team are striking out so often (21% of all plate appearances) in the Eastern League that I had to change the y-axis scale on this graph:

Until last week, home teams were generally improving their strikeout-to-walk ratio, while visiting teams were getting worse. Akron appears to enjoy one of the more significant home field advantages in all of baseball. They strike out nearly three and a half times for every walk on the road, but they enjoy a 1.84 strikeout-to-walk ratio at home. Akron’s Kevin Kouzmanoff is one of the few minor leaguers to maintain a .400 batting average this late into the season. He’s having a great year by any standard, but he has only struck out four times at home this season and his .469 average at Akron certainly helps. Akron pitchers are also experiencing different results at home and on the road (see, for example, Adam Miller).

Southern League

There is significant week-to-week variance here, but I think the trend is clear:

West Tenn and Chattanooga are enjoying the biggest home field advantage in strikeouts and walks. And just to prove every rule has an exception (especially when dealing with relatively small samples), the 20-29 Huntsville Stars are walking in 10.8% of their plate appearances on the road and only 8.4% of their plate appearances at home.

Texas League

In my earlier article, I noted that the home field advantage did not appear to be an issue in the Texas League. This is starting to change:

The two most notable splits are happening in Frisco and Wichita. Wichita hitters, for example, are only striking out 1.23 times per walk at home and 2.22 times per walk on the road. This piece of information might help explain why top Royals prospect Alex Gordon is hitting .359 at his traditionally pitcher-friendly home park.

In sum, there is mixed evidence in support of the “getting to know the local umpires” hypothesis. The home field advantage for striking out and walking is fairly robust across these five leagues, but the trends over time are not consistent. Four of the five leagues profiled here did experience increased home field advantages in the month of May, but the size of the differences were not similar across leagues, and not always significant. I will present more information and focus on the lower minor leagues later this week, but until then please don’t hesitate to contact me with questions and ideas.

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