Minor Matters: Home Field Advantage in the Lower Minor Leagues

Regular umpires will return to minor league ballparks next week, after the Association of Minor League Umpires (AMLU) strike was recently resolved with a new six-year contract.

This seems like a good time to wrap up the discussion of how the replacement umpires may have changed minor league baseball over the past two months. This week I am focusing on the “getting to know the local umpires” hypothesis and trying to figure out if home teams have an advantage because they are learning local umpires’ tendencies. On Monday, I examined trends in the Double-A and Triple-A leagues and concluded that there was mixed evidence in support of this hypothesis. Today I will look at the High-A and Low-A leagues in search of evidence that the home teams are learning to take advantage of the local umpires’ strike zone judgments.

California League
There is no evidence that home teams are gaining an advantage as the season progresses in the California League:

That said, the game has significantly changed in this traditional hitter’s league. Run scoring is down, strikeouts are up, and walks are down (slightly) relative to the league’s three-year average and the numbers at this time last year. The same is true in nearly every minor league this season and is probably a reflection of the replacement umpires’ more generous strike zones. This is not necessarily a good thing or a bad thing; it’s just a difference that’s worth paying attention to. If the AMLU umpires return in June, expect minor league pitchers’ earned run averages to rise. This could be especially salient for pitchers with unusually low walk rates or high strikeout rates during the first two months of the minor league season.

Carolina League
The early results here also do not support the “getting to know the local umpires” hypothesis.

The Lynchburg Hillcats have the most extreme home and away splits in the Carolina League.

Florida State League
The Florida State League is traditionally a pitcher-friendly league, reflected in the league’s high strikeout-to-walk ratio. The overall home and away splits are significant, but it does not appear that this has changed as the season progressed:

The most extreme home field advantages have occurred at Vero Beach, Jupiter, Brevard County, and Lakeland. Lakeland Tigers hitters are walking twice as often at home than on the road, and they are striking out less often at home, too. It’s worrisome to think Tigers third base prospect Wilkin Ramirez might actually have worse strike zone judgment than what is reflected in his strikeout and walk totals this year.

Midwest League

There is a difference at home and on the road, but it’s not significantly different from the difference in the Midwest League last year. Even major league hitters strike out less and walk more at home than on the road, so we cannot simply blame local replacement umpires for the small differences we are seeing in theMidwest League. More importantly, the difference seems to be getting smaller over time rather than growing with experience.

South Atlantic League
The differences at home and on the road are more clear in the South Atlantic League, but there is no strong evidence of a growing home field advantage. The “getting to know the local umpires” hypothesis does not seem relevant in either of the low Class-A leagues.

Lake County, Kannapolis, and Charleston all appear to be enjoying some form of a home field advantage, but Greensboro leads the league in this respect. Greensboro hitters are striking out in 23.9% of plate appearances on the road and only 17.8% of plate appearances at home. Their pitchers are producing similar differences in home and road results; see Marlins prospect Chris Volstad‘s home/road splits, for example.

In sum, there is very little evidence that the players in these leagues were learning to take advantage of local umpires’ strike zone judgments. This result contrasts with the trends in the upper minor leagues. One possible explanation for these two conflicting sets of results is that the players in the Class-A leagues lack the experience to take advantage of contextual factors such as local umpires’ tendencies. Instead, these younger players may be more focused on more foundational skills such as refining a secondary pitch on the mound, working on their pitch recognition at the plate, or learning to smoothly turn a double play in the field.

There is some evidence that players are taking advantage of their knowledge of local umpires in the minor leagues, but this is not consistent across all contexts. Instead, it appears that these differences are more salient in the upper minor leagues and in certain cities. There might be stronger evidence for this effect if minor league teams had more time to learn about the local umpires, but I expect any growing home field advantages to dissipate if the AMLU’s travelling umpire crews return to work next month.