Unfuzzing the Strike Zone

Sports Info Solutions has been tracking every pitch thrown in Major League Baseball since 2002, and since the beginning, those pitches have been hitting the strike zone less and less frequently. You can check the tumbling year-over-year numbers over on our pitch-level data leaderboard, but if you want to spare yourself a click, I pulled them into the graph below. It paints a damning picture of the command of today’s stuff-over-stamina, throw-it-hard-before-your-elbow-explodes pitchers. Don’t go near this graph if you’re on roller skates:

If you ever feel the need to shake your fist at young pitchers and mutter about loud music and fastball command, this is the graph for you. SIS has documented the percentage of pitches that hit the strike zone dropping from the low 50s to the low 40s over the last 20 years. Combine that with the game’s ever-increasing focus on velocity and stuff, and you’ve got a nice, tidy narrative: today’s pitchers are too focused on throwing hard to know where the hell they’re throwing the ball. However, the truth is a bit more complicated. It’s important to keep in mind that the SIS numbers come from real life human beings who analyze video to track pitches, while the friendly robot that powers Statcast has its definition of the strike zone set in digital stone.

The robot invasion began more than 20 years ago. Major League Baseball started using QuesTec to grade the ball-strike calls of umpires in 2003, then upgraded to PITCHf/x in 2008 and Statcast in 2015. Ever since 2003, umpires have been getting better and better at calling the zone the way the robots want them to, which has resulted in more called strikes. That makes it sound like the strike zone has gotten bigger, but something else has actually been going on. The edges of the zone have gotten less fuzzy, which in turn has subtly reduced it in size — sort of. Jeff Sullivan was writing about this as early as 2016. It’s not that pitchers suddenly couldn’t throw a strike. Here’s the same chart with Baseball Savant’s data included:

See what I mean? That red line has stayed roughly the same, going all the way back to 2008. Statcast doesn’t think today’s pitchers are just throwing the ball wherever they want with no regard for the strike zone. In fact, according to Statcast, there’s a slight trend towards more pitches in the zone and more pitches over the heart of the plate. Before the pitch tracking era, the edges of the zone were big and fluffy. It went both ways, with balls that should’ve been called strikes and strikes that should’ve been called balls, but understandably, only the former affected the size of the zone. Let me show you what I mean. Below are two representations of the strike zone. The one on the left is perfectly crisp, while the one on the right is fuzzy:

The zone on the left is essentially what Statcast sees. Every pitch either touches the zone or it doesn’t, with no gray area. The pink pitch is always a ball and the blue pitch is always a strike. The zone on the right is meant to represent an actual umpire’s strike zone, which, as Russell Carleton recently wrote for Baseball Prospectus, is essentially probability-based: a pitch in X location has an X percent change of being called a strike. The edges are fuzzy in both directions; sometimes the blue pitch will end up as a ball and sometimes the pink pitch will end up as a strike.

Now let’s say that you’ve been asked to draw a representation of the strike zone. If your conception of the zone is purely mathematical and your representation includes any pitch that gets called a strike more than 50% of the time, then even for that fuzzy zone on the right, you’d still draw the picture of the zone on the left. But that’s not what you’d draw if you were a human being who watched the game. Any practical diagram of the zone is likely to end up slightly larger than the rulebook zone in order to account for that pink strike. The fuzzier the zone gets, the greater its effective size. Back in 2019, Eli Ben-Porat demonstrated this by creating different diagrams of the zone based on your threshold. The big blobs show the zone if you define it as a pitch getting called a strike 50% of the time, while the small blobs have an 80% threshold:

Twenty years ago, if a pitch five inches off the plate could end up as a called strike, it only made sense for the fine people at SIS to mark it down as hitting the strike zone. Naturally, year over year, as umpires got better and the edges of the zone firmed up, those wider pitches had a much lower chance of being called strikes, so they got marked down as balls and the zone shrank. In effect, Statcast’s zone is prescriptive, as it’s literally the rectangle that umpires are expected adhere to, while the SIS zone is descriptive, showing the edges of the zone that umpires are actually calling. If you’ve ever stayed up into the wee hours of the morning wondering how the strike zone was similar to a dictionary, you can now rest easy.

I’ve got a couple ways to show this effect practically. Let’s start with our trusty 10-by-10 FanGraphs heat maps. The GIF below shows the called strike rates for the entire league in 2007 and 2024. In other words, it shows what percentage of the time a ball in each square would have been called a strike. When you compare the two maps, the first thing you’ll notice won’t be the size of the zone, but the shape:

Over the years, the zone has gotten taller and skinnier. At this point, we should note that that first QuesTec system was an overhead camera that only judged umpires on their calls on the edge of the plate. It’s no coincidence that as the zone shrank back to the actual width of home plate, it also got taller. I wrote about that phenomenon a few weeks ago, but don’t let that distract you from the overall size difference. The map below shows the differences in each square:

There’s a whole lot of pink inside the zone and a whole lot of blue outside the zone. According to research from Jon Rogele, the increased likelihood of called strikes below the zone actually increased the size of the zone from 2008 to 2013. Still, although the zone definitely got taller, what’s important for our purposes is that it got much, much more consistent. Here’s how that consistency translates into a smaller effective zone. Below are two pitch charts that show every single called strike earned by the pitching staff of the Yankees. On the left is 2008 and on the right is 2024:

Look at the big, unkempt ball on the left. It’s straight out of Katamari Damacy. Back in 2008, the zone was so much shaggier. Anything relatively close to the edge of the plate had at least a chance of being called a strike. The 2024 zone on the right is a bit more robust at the top and bottom, but it’s pretty much the exact shape of the rulebook rectangle. It has defined edges with straight lines and corners, and if Tom Glavine ever saw it, he’d have nightmares for weeks. In case you’re curious, the same shagginess also applies to called balls:

The edges of the zone just used to be much more permeable. Tightening up the edges went both ways. I combined the two charts for each season to show how much bigger that fuzzy area, in which a pitch could end up as either a ball or a called strike, used to be. There was so much more gray area back in 2008:

I’ve shown this to you on a league-wide level and a team level, but the effect is so pronounced that the heat maps even let us see it on an individual player basis. The heat maps below show Zack Greinke’s called strikes in 2008 and 2019, the first and last seasons when he hit 200 innings pitched. Once again, the 2008 heat map is on the left:

Look how wide the map on the left is. One of Greinke’s hottest spots for called strikes was fully off the plate toward his arm side. Toward the end of his career, those calls simply weren’t there anymore.

If you’ve ever heard someone complain about an umpire’s strike zone, you’ve probably heard them say they care less about its size and more that it stays consistent. Personally, I’ve never put much stock in that particular argument, but with these various visualizations of the zone, I’ve been hoping to show you how that increased consistency has affected things. Umpires aren’t just missing less often according to the Statcast zone, they’re missing much smaller, on the true borderline calls. So the next time you hear someone say that today’s flame-throwing single-inning relievers have no idea where the ball is going – or that umpires have never been worse – feel free to set them straight.