Are the Minor League Replacement Umpires Biased?

When Delmon Young was called out on strikes and launched his bat at an umpire in Pawtucket nearly two weeks ago, his anger was directed towards a replacement umpire. Young’s actions were foolish and reckless, but he is certainly not the only minor leaguer frustrated by umpires’ decisions this year. Unionized minor league umpires have been striking all season long and the minor leagues are relying on local high school and college umpires this season.

Players and managers have been instructed to “be patient” with the replacement umpires, but a number of baseball people have commented on home plate umpires’ inconsistency in calling balls and strikes. Some some managers are specifically complaining about unfair “home town treatment” because the umpires are hired locally. One anonymous manager claimed that “every game there were two separate strike zones—one for us and one for them.”

New Orleans Zephyrs manager Tim Foli reportedly charged a crew of umpires with hometown bias and suggested “we’re better off in a lot of situations just having one of the coaches stand behind the mound or letting the catchers call (balls and strikes).”

Do the local umpires really put visiting teams at a disadvantage? The frequency of strikeouts and walks for home and away teams may provide some insight into this question. If umpires are giving home teams the benefit of the doubt, we would expect lower strikeout rates and higher walk rates for home team batters. Here is a breakdown of this season’s strikeout and walk rates for home and away teams in all of the full-season minor leagues:

			   HOME		   AWAY		Advantage
International	AAA	18.7	8.7	19.6	8.0	+0.29
Pacific Coast	AAA	17.1	9.4	19.5	7.8	+0.69
Eastern		AA	20.1	7.9	21.6	7.6	+0.28
Southern	AA	20.5	8.9	21.1	8.8	+0.08
Texas		AA	18.4	8.4	19.7	8.8	+0.05	
California	A+	20.8	8.7	20.8	8.2	+0.16
Carolina	A+	18.5	9.0	20.5	8.9	+0.25
Florida State	A+	20.8	8.4	21.0	7.5	+0.31
Midwest		A	20.2	9.0	20.4	8.6	+0.13
South Atlantic	A	21.2	8.3	21.8	7.8	+0.25

The ‘Advantage’ figure is a rough estimate of the differences in home and away teams’ results at the plate. It is simply the K/BB ratio for the home team subtracted from the K/BB ratio for the away team. A positive number indicates a better strikeout-to-walk ratio for the home team hitters.

As you can see, the home teams are generally striking out less often and walking more often in comparison to the away teams this season. The size of the differences varies across leagues, but the direction of the effect is consistent across leagues. The Texas League, where visiting batters are walking more often than the home batters, is the lone exception.

On the surface, this statistical evidence appears to support the complaints about hometown bias. We cannot generate a strong claim about the effects of the replacement umpires, however, without looking at how these results compare to past season’s results with the AMLU umpires behind home plate. I’m collecting the data we need to make this comparison, but this is very much a work in progress and at the moment I only have April 2005 results from the International League and Pacific Coast League.

			   	   HOME		   AWAY		Advantage
International	AAA	2005	17.7	7.7	18.4	8.1	-0.01
International	AAA	2006	18.7	8.7	19.6	8.0	+0.29

Pacific Coast	AAA	2005	17.8   10.1	19.2	9.8	+0.20
Pacific Coast	AAA	2006	17.1	9.4	19.5	7.8	+0.69

In both leagues, the recent differences in home and away results are much greater than last season’s differences at this point in the season.

The differences in the Pacific Coast League are particularly striking. Home team batters have a K/BB ratio of 1.82 while visiting batters have a K/BB ratio of 2.50. I wish I could say the situation is getting better after a period of initial adjustment, but that doesn’t appear to be the case. Over the first four weeks of the season, the home-field advantage did not deteriorate:

The season is still young and we need to collect more information to better understand the effects of the replacement umpires, but so far all the evidence all points in the same direction. Home team batters appear to have an advantage at the plate. Furthermore, there may be other effects of the new umpiring population that are more difficult to quantify or assess with traditional statistics such as strikeouts and walks. It is clear, however, that we need to be more careful than ever when analyzing minor leaguers’ performance. The initial evidence suggests an appreciation for a prospect’s home and away splits may be particularly important this year.

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