Posted by: Peter | October 7, 2009

Strike Zone Follies

Twins Geek remarks on Randy Marsh’s strike zone last night, and says that “it seems like a shame that an umpire’s very bad day could affect such a critical game”. I certainly had the same impression–although the one indisputably bad call, missing Inge getting hit by a pitch, had nothing to do with strike calling. At one point I texted my friend: “mad inconsistent strike zone tonight”. But while I was sleeping off that epic win, my pitchf/x database got updated with last night’s game, so I figured I should take a look at the data:

Dave Marsh's strike zone, October 6th 2009

Randy Marsh's strike zone, October 6th 2009

These are only the balls that Marsh actually called–i.e., the ones that weren’t swung at. Green means it was called a strike, and black means it was called a ball.

We can certainly see here that Marsh’s strike zone was small, which according to the game announcers is the norm for Marsh. But I’m not sure it was actually all that inconsistent–except maybe in the 11th, when it looks like he started calling more strikes.

Here’s another way of looking at it: a graph that plots all of the called pitches from last night on one graph:

Randy Marsh's calls, October 6th 2009

Randy Marsh's calls, October 6th 2009

The gray box is the “rulebook” strike zone (with the top and bottom based on averaging over different players). The dark lines are my estimates of the “true” strike zone. I won’t go into the details, but basically I use a regression model to figure out the point where a pitch has exactly a 50-50 chance of being a ball or a strike. Above, I show both an estimate for a normal rectangular strike zone and for an elliptical zone. My research suggests that real strike zones aren’t actually rectangles, so the ellipse may give a better sense of how pitches are actually called.

Finally, let’s look at all of Randy Marsh’s calls this year and last year (the period for which I have pitch data). Was last night’s strike zone unusual?

Randy Marsh's calls in 2008-9

Randy Marsh's calls in 2008-9

Doesn’t look like it. You can barely see the rulebook zone on this graph, but Marsh just always seems to call a smaller, higher strike zone than the rulebook, and that’s what he did last night, too. And it turned out to be all the strike zone the Twins needed.



  1. […] determine what counts as a quality strike, I’ll use the strike zone model I described in this post. Basically, I assign each pitch a value between 0 and 1, indicating how likely that pitch is to be […]

  2. […] DiMuro’s strike calls in yesterday’s game. After my last post, I was prepared to make yet another post on rulebook-versus-true strike zones and the bias of fans. But this, time, you’ll get no […]

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