Ballpark Strike-Zone Factors

So, look, I feel like I need to explain why I’m looking at something you’ve probably never even thought about before. Obviously, I write a lot about pitch-framing, and about the different strike zones that different players get. Some time back, I was writing about Zack Greinke‘s favorable zone with Milwaukee. As such, I wrote about Jonathan Lucroy‘s receiving, and I wrote about Martin Maldonado’s receiving. Afterward, I received an interesting idea from a player:

I’m convinced the background in Milwaukee affects the home plate umpires’ depth perception and expands the zone down and away. Is there a home road split for both of the catchers? Maybe that will make some sense for my possibly distorted perception of balls and strikes in Milwaukee.

Not something I’d considered. Made sense, in theory. Perhaps a hidden park factor was inflating the framing numbers. Maybe Milwaukee is like the Colorado of expanding the strike zone. For whatever reason I never got around to researching this, until this afternoon, when I compiled the relevant data.

Per usual, I’m thankful for the existence of Baseball Savant. I looked at called-strike rates both inside and outside of the zone for 2012 and 2013, for every team, for hitters and pitchers, home and away. I combined the data that needed to be combined and calculated ratios of home strike rates to road strike rates. It’ll all make more sense with the data and a bit of explanation, so let’s just get to that part now. Here’s a table of home-to-road called-strike-rate ratios:

Team In-Zone Out-Of-Zone
Angels 1.03 0.98
Astros 1.01 0.96
Athletics 1.00 0.99
Blue Jays 0.98 0.98
Braves 1.00 1.02
Brewers 1.01 0.99
Cardinals 0.99 0.97
Cubs 1.00 1.03
Diamondbacks 1.00 0.97
Dodgers 0.99 0.97
Giants 1.00 1.00
Indians 1.02 1.02
Mariners 1.01 1.06
Marlins 0.99 1.01
Mets 1.00 0.96
Nationals 1.00 1.01
Orioles 0.99 0.93
Padres 1.02 1.06
Phillies 1.01 1.00
Pirates 1.00 1.01
Rangers 0.97 1.06
Rays 1.02 1.01
Red Sox 1.00 0.97
Reds 1.02 1.01
Rockies 0.98 0.96
Royals 1.00 0.98
Tigers 1.01 1.04
Twins 1.01 0.99
White Sox 0.97 1.03
Yankees 1.00 1.07

It ought to be sortable! What do we find? Let’s look at the Brewers and Miller Park first, since they inspired the whole idea. The last two years, in Miller Park, called strikes have been up one percentage point within the zone. Outside of the zone, though, they’ve been down a tiny bit. The difference is small, but overall I’m left unconvinced by the player’s theory — it doesn’t look like Miller has a huge park factor at play. Perhaps there’s slight strike inflation, but it doesn’t explain Lucroy and Maldonado away.

Other numbers are more interesting. In Angels home games, 90.0% of pitches taken in the zone have been called strikes. In Angels road games, 87.8% of pitches in the zone have been called strikes. Yet when it’s come to pitches out of the zone, umps have been a little more stingy in Anaheim. I’m not sitting here, offering explanations. I’m just offering numbers and you can try to come up with your own explanations.

At the other extreme, Rangers and White Sox home games have featured reductions in in-zone called strikes. Yet they’ve also featured slight increases in out-of-zone called strikes. What about ballparks where both factors are either above or below 1.00? St. Louis, Baltimore, Los Angeles, Toronto, and Colorado have reduced both kinds of called strikes. Seattle, San Diego, Cincinnati, Cleveland, Tampa Bay, and Detroit have increased both kinds of called strikes.

In Yankee Stadium, 16% of pitches taken out of the zone have been called strikes. In Yankees road games, that drops to 15%. In Camden Yards, just under 14% of pitches taken out of the zone have been called strikes. In Orioles road games, that jumps to nearly 15%. Do teams bat more lefties in New York? Does that open them up to the lefty strikes away off the plate? I haven’t a clue. Again, I’m just playing the part of messenger.

In Coors Field, in-zone strikes have been down 2.1 percentage points, and out-of-zone strikes have been down 0.5 percentage points, for a sum effect of -2.6 percentage points. That ties the park with US Cellular. Flipping around, we have Petco Park, where in-zone strikes have been up 1.8 percentage points, and out-of-zone strikes have been up 0.9 percentage points, for a sum effect of +2.7 percentage points. Next-closest is Angel Stadium, at +1.9. The Padres have come away recently with some surprisingly terrific pitch-framing numbers. Might some of that have been the ballpark, somehow? I don’t know nearly enough to be able to answer that, but I’m at least open to the concept. There can be park factors for anything.

The player at the start had an idea about Miller Park. After digging in, I don’t see a strong effect, but I’m glad to have had the idea provided to me, and there seem to be stronger effects elsewhere. Is there something about San Diego that makes an umpire more likely to call a strike? Is there something about Colorado or Chicago that does the opposite? These are things to investigate, as never does the tunnel of questions end. It gets narrower in places, sure, but we’ll always be able to keep going.

Jeff made Lookout Landing a thing, but he does not still write there about the Mariners. He does write here, sometimes about the Mariners, but usually not.

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8 years ago

What’s the relative size of these numbers and pitch framing numbers? If they’re all signal and no noise, how much of pitch framing could they explain?

On the other hand, what’s the SD of those numbers, and what would you expect just from noise?

8 years ago
Reply to  sambf

I second this question. It looks like nearly all noise to me, given that:

1) If you were to bucket the parks into above/below average in/out of the strike zone (in a 2×2 grid) it would be pretty evenly distributed. This would contradict any overall strike-factor hypothesis.

2) The ratios for in-zone strikes are more closely bundled than out-zone strikes. This makes sense if these numbers were randomly generated because the denominator is smaller in the out-zone strikes calculation.