Do Umpires Favor the Home Team?

So, I've been playing around with the pitch locations and related umpire calls in order to see if there's a umpire bias. Here, I'm just looking at home vs. visiting team, and it is only one game. Still it is interesting. The first graph is for the visiting team's pitcher, Dereck Rodriguez ("Pudge" Rodriguez's son), while the second is the home team's pitcher, German Marquez (from last night's game between the Giants and Rockies). The pitch locations are for called strikes and balls -- in other words, pitches at which the batter didn't swing, so the umpire had to decide whether the pitch is a strike or a ball.

There's a statistical test for this, but a quick glance suggests that the home team received better calls than the home team did. This, of course, is only one game, so we shouldn't generalize. I will return in a later post with a more substantive analysis that will examine umpire patterns for all of major league baseball, dating back until 2008. I first need to figure out how to store all that data, however.