Pitching to the Ump
A couple of days ago, Ben Walker of the Associated Press reported that teams are scouting umpires. I decided to check on the data to see whether pitchers have been changing their approach based on the umpire.
Umpires' zones vary from game to game, yet some umpires develop reputations around the league for perhaps calling the high strike or maybe sleeping next to an ice bucket. For most umpires, the PITCHf/x system has recorded enough data for an analyst to create a strikezone probability distribution. I'm not going to name any specific umpires, since that might come off like I was trying to evaluate them, which I'm really not, but I did make these probability distributions for the league on average as well as for each umpire, controlling solely for batter handedness. I hypothesized that the difference in a pitcher's expected called strike percentage without controlling for the umpire vs. the same pitcher's expected called strike percentage while controlling for the umpire could be attributed to the pitcher's knowledge of the umpire.
I found that, given the internal consistency in the data, there is certainly some skill to this effect, but the magnitude of the effect was small I think. , Livan Hernandez, who you may recall was on the same page as Eric Gregg back in 1997, actually has, by the numbers, done the worst job of adjusting for the umpire, as his pitches were 4% less likely to be called strikes given his distribution of umpires than given an average umpire. While the reliability tests I ran showed that Livan was consistently below average at "pitching to the umpire," I dug deeper, and I can't shake the feeling that luck plays a huge part of it. Sorting through umpires, I couldn't find any difference in Liva's approach. But maybe that's the problem. His approach is consistent, and it's the umpires who change. Here, I present a pair of charts displaying data on Livan Hernandez pitching to an umpire who has called a couple of his games.
I've taken the difference between the average strike zones and a given umpire's strike zone. Blue areas represent spaces where this umpire calls fewer strikes than average, and red areas represent spaces where an umpire is more generous. I made a density estimation to model the distribution of Livan Hernandez's pitches against batters of each handedness, and then plotted a contour line that displays where he's generally pitched over the last few years. I finally plotted the locations of the individual pitches that Livan has thrown with this specific umpire calling the game.
It turns out that against righties, this has been Livan's favorite umpire. The ump does a great job calling pitches below the knees, and he gives pitchers the down-and-away strike, which is right in the center of where Livan generally pitches. So Livan, who has been 7% more likely to have a pitch called a strike with this umpire behind the plate than an average ump, hasn't actually done anything different. This ump just suits his style.
Meanwhile, against lefties, Livan pitches exclusively away, and he hasn't changed up his approach, even though this umpire does not tend to give pitchers that call. So in this way, Livan, without doing anything differently, is failing to "pitch to the ump."
This type of information could also be of value to a manager in deciding whether to throw a sinkerballer who pitches down in the zone or a power pitcher who goes up the ladder. I don't think that pitchers should, or do, change their approach much based on the umpire behind the plate. However, every inch counts, so the information can't hurt.