At one point or another, most of us have done the thing where we go to the refrigerator in search of a snack, decide nothing looks appealing, close the door, then come back 15 minutes later to check again and somehow feel annoyed when the contents remain unchanged. It’s a near-universal experience despite the illogical nature of the whole thing. And when we relate this experience to others, it’s always the refrigerator, even though we could just as easily choose to re-check a cabinet or the pantry. But I think this is where we do get some credit for being slightly logical. The contents of a refrigerator are far more transient than the dry and canned goods stored elsewhere in the kitchen. The fridge is where we keep the perishables, the food that by definition isn’t meant to last long. Food in the refrigerator comes and goes, rots and gets tossed, all at a much faster rate than elsewhere in the kitchen.

Park factors work a little like a refrigerator. They present a single value that contains within it the influence of several different components that vary from park to park, much in the way my refrigerator is two-thirds beverages and cheese, while yours probably has fruits and veggies and maybe some leftover ham from Easter that you should definitely throw away. Some of the components captured by park factors are static and easily measured, like surface dimensions and wall height. They’re the condiments that remain consistently stocked in the fridge door.

But sometimes you throw open the door to a park’s refrigerator and get whacked in the face with a stench of unknown origin. And that stench becomes all the more potent as it mingles with a to-go box of leftover Thai and a carton of milk growing more questionable by the day. Likewise, wind speeds, the daily dew point, and the angle of the sun at different points relative to the solstice all fluctuate and interact in a way that a scientist with the right expertise could tease out and quantify, but that remain a bit fuzzy to the casual observer.

It was these squishier components of park factors, the ones that ebb and flow as weather cycles in and out and the seasons change, that sparked my curiosity about how park factors might vary over the course of such a long season. Traditionally, park factors are calculated over multiple full seasons of data (though sometimes single-season park factors are useful for capturing more recent trends), and that’s not just a sample size consideration. A full season of data is needed to ensure a balanced schedule where every opponent faced on the road is also faced at home and vice versa. This ensures that when comparing runs per game at home to runs per game on the road, the team quality is consistent in both subsets. Read the rest of this entry »

Last week, I investigated the increasing divergence between the way pitchers approach same-handed and opposite-handed batters. I learned that pitchers across the league are varying their arsenals more and more every year. But that was a broad look, and I had some follow-up questions. Mainly, who specifically? Which teams? Which players? And how? Today, I’ll provide some answers.

As a refresher, I calculated what I’m calling “adaptation score” by comparing how frequently a pitcher uses his top-two offerings, both against same-handed and opposite-handed batters. Adaptation score is simply the difference between how frequently a pitcher throws his two best pitches when he has the platoon advantage and how often he throws those same two pitches when the batter has the edge. I split the data up by teams to see who was driving the move. First, we’ve got the five most and least adaptable teams in 2025:

Not much to see here. The Dodgers’ being on the bottom might suggest that adaptation is bad, even. But truthfully, there’s a big element we’re missing in looking at the data this way: personnel. Changing who’s on your team, even if you have the same philosophy, can change how you score in this metric. The Dodgers were in the middle of the pack last year when it came to adaptation score. Then they overhauled their pitching staff and ended up here.
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If you are familiar with Andrés Muñoz, the baseball player, you may know that he is good. It may be enough for you to simply witness and bask in his elite performance, and question it no further. (Rarely are we so content here.) You may not realize he is unusual; you may not care. Often in baseball, being good and being unusual go hand in hand. This is a short exploration, albeit one preceded by an exorbitantly long prologue, of why Muñoz is good and unusual.

If you are familiar with FanGraphs, the baseball website, you may know about approach angles. If not: A pitch’s approach angle is the three-dimensional angle at which it crosses the front of home plate. Broken down into its two-dimensional vectors, it becomes vertical approach angle (VAA) and horizontal approach angle (HAA).

VAA is a description of pitch shape and thus depends on other physical attributes of the pitch — namely, its velocity and acceleration in all three dimensions. While representing the most distilled measurements of a pitch’s movement through space, the velocity and acceleration vectors themselves are functions of release height, release angle, release speed, spin rate, spin axis, spin efficiency… it goes on. VAA, as it happens, is very sensitive to pitch height. Reporting a pitch’s average VAA is not especially meaningful without either providing locational context or stripping it of that context all together.

To accomplish the latter, I developed VAA Above Average. It’s a simple recalculation that communicates a fastball’s flatness or steepness irrespective of pitch height. Through this it’s much easier to see that, for example, flatter VAAs induce higher swinging strike rates (SwStr%) at all pitch heights compared to steeper VAAs (forgive the half-baked visualization):

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I’ve been writing about the strike zone for a few years now, and if there’s been one overarching theme to my work so far, it’s the inescapable takeaway that umpires are excellent at what they do. When Major League Baseball introduced PITCHf/x in 2008, umpires got 84.1% of ball-strike calls right according to the Statcast strike zone. Over the intervening years, while ever-nastier stuff and a revolution in pitch framing had made their jobs harder and harder, umpires did nothing but get better. Accuracy broke 92% in 2021 and inched its way toward 93% over the next two seasons. That trend of improving every year finally changed in 2024.

As I wrote yesterday, last season marked the first time that umpires got worse rather than better. That’s interesting enough on its own, but right when it was time to wonder whether they’d gotten as good as they could get, the rules of the game changed. Over the offseason, a new labor agreement included a change to the way that umpires are assessed by the league. The grading got much tighter, reducing the buffer around the edges of the strike zone from two inches on the outside of the zone to three-quarters of an inch on either side. The strike zone is the same, but umpires are being judged much more tightly. Let’s dive into the numbers and see what looks different so far this season. Here’s a graph that shows overall accuracy in every season of the pitch tracking era.

The yellow line shows overall accuracy, and it’s ticked back up from 2024. Even though it’s early in the season, a time when umpires are at their least accurate, they’re still doing better than they did last year. Accuracy fell from 92.81% in 2023 to 92.53% in 2024, and is now back up to 92.63%. In fact, if you look only at March and April stats – which is more fair, because umpires are worse earlier in the season – you’ll find that umpires just had their best opening month of the season ever. They called 82% of pitches in the shadow zone correctly. Read the rest of this entry »

Back in January, I wrote an article called “Unfuzzing the Strike Zone.” The premise was pretty simple. As umpires have grown more accurate, as the edges of the strike zone have gotten clearer and more distinct, the strike zone has effectively gotten smaller. Misses go both ways, but there’s a big difference between an incorrectly called ball and an incorrectly called strike. Calling a pitch inside the zone a ball doesn’t shrink the effective size of the zone, but calling a pitch outside the zone a strike does make it bigger. As long as a pitcher knows it’s possible to get a strike call out there, they’ll consider it part of the zone. Little did I know that as I was writing that article, Major League Baseball was preparing to test its exact premise.

The strike zone has steadily shed its fuzz over the past 23 seasons, but on Thursday, Jayson Stark and Ken Rosenthal reported in The Athletic that the league has decided to break out a sweater shaver. Over the offseason, the Major League Umpires Association came to a new agreement with MLB. Part of the agreement included tightening the standards by which ball-strike calls are graded.

Umpires used to have a two-inch buffer around the edge of the strike zone, meaning that if they’d missed a call by fewer than two inches outside the zone, the call would still go down as correct in their assessments. Having that buffer is necessary because calling balls and strikes is extraordinarily difficult. It’s extremely rare for an umpire to get every call right even in a single game. The new border is just three-quarters of an inch on either side. The league is demanding a less fuzzy strike zone. Read the rest of this entry »

Every winter, pitchers step off the mound and into the lab. Sure, not every pitcher is in a wind tunnel with a high speed camera from October to January, but enough are that everyone seems to reap the benefits. You’ve got the sweeper, the kick change, the rise of the splitter, new fastball shapes – you name it, someone has tried it recently. That means that every year, I spend the first month or two of the season catching up to the new hot thing going on in the world of pitching.

But I have to level with you: On the whole, things haven’t changed as much this year as I expected. That won’t stop me from walking through what has changed, though, and the first shift to highlight is a subtle one – we’re seeing more bendy sliders and fewer gyro offerings:

Why? Two things are happening. First, sweeping sliders do better against same-handed batters, so pitchers are choosing that as their secondary of choice when they have the platoon advantage. In 2021, 2.6% of pitches that righties threw to righties were sweepers. In 2025, that number has ballooned to 10.7%; it’s 10.9% for lefties against lefties. Usage is less than half as high when opposite-handed batters are at the plate.

Meanwhile, “regular” sliders are on the decline when pitchers have the platoon advantage. That makes good sense – they’re just throwing sweepers instead. And when pitchers aren’t facing same-handed batters, neither slider is particularly great; pitchers are staying away from both, more or less. That means that the traditional, gyro-spin slider is declining in prevalence overall. Read the rest of this entry »

This season is the third since the implementation of a spate of significant rule changes across the majors. Along with a pitch clock and limits on defensive positioning, a limit on disengagements (read: pickoff throws plus idle standing around) combined with slightly larger bases gave runners a collective green light. With fewer throws to first, bigger targets to slide into, and more predictable pitcher deliveries thanks to the clock, stealing a base got much easier overnight. In 2022, the last year of the old rules, the majors saw 2,486 steals across the entire season. In 2024, that number surged to 3,617 steals. Even better from an offensive perspective, the stolen base success rate jumped from 75.4% to 79% over that span.

The first year of the new rules was all about experimentation. Some players ran wild – Ronald Acuña Jr. more or less took off every time he could. Meanwhile, the Giants stole just 57 bases as a team, fewer thefts than the previous year, when those steal-boosting rules weren’t yet in effect. None of that seems particularly surprising to me; when new rules of this import are added to the game, every team will scramble to figure out how to change their own behavior to benefit. There were a ton of moving parts, and many teams took a simple approach: keep stealing more and more until it starts to fail.

The 2024 season was the year of the defensive reaction. Teams attempted 209 more steals in 2024 than they did in 2023, but only succeeded on 114 of those extra steals. The aggregate effect was a lower success rate on marginally more attempts. Catcher pop times improved, pitchers threw over more often, and defenses were more attentive to baserunners in general. That brings us to 2025, and in the early going, it looks like the baserunners are continuing to push the envelope:

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To every thing there is a season, and a time to every purpose under the heaven:
A time to be born, and a time to die; a time to plant, and a time to pluck up that which is planted;
A time to kill, and a time to heal; a time to break down, and a time to build up;
A time to weep, and a time to laugh; a time to mourn, and a time to dance;
A time to cast away stones, and a time to gather stones together; a time to embrace, and a time to refrain from embracing;
A time to get, and a time to lose; a time to keep, and a time to cast away;
A time to rend, and a time to sew; a time to keep silence, and a time to speak;
A time to love, and a time to hate; a time of war, and a time of peace.

– Ecclesiastes 3:1-8

On Thursday, Toronto Blue Jays outfielder Nathan Lukes welcomed a baby named Jett into the world. That same day, teammate Daulton Varsho was expecting to welcome his own baby. When I read the news, I did what anyone would do. I thought, “How wonderful for the Blue Jays,” and then I asked the internet to do some math for me.

Well that’s fun. Nine months before Lukes and Varsho became fathers, it was July 16, 2024. That date may ring a bell, because it was also the date of the All-Star Game. Lukes wasn’t in the majors at the time, but clearly, both players had very productive All-Star breaks. I decided it was time for a full investigation. Do baseball players make all their babies during the All-Star break? Read the rest of this entry »

Last year, David Appelman and I set about injuring a ton of players. Wait, that doesn’t sound right. Let’s try this again. Last year, David Appelman and I developed a method to use our depth charts projections to simulate how much injuries to the league’s top players might affect each of the teams in baseball. Today, we’re updating that article for the 2025 season. I’ll also present some research I’ve done into how these injury-aware depth charts compare to actual historical seasons.

First, a review of the methodology is in order. If you don’t need an update, or if you simply want to get right to the data, you can skip ahead; the results section is clearly labeled below. We decided to simulate depth by first removing the top X players from a team’s depth chart and then reallocating playing time to fill in the missing plate appearances or innings pitched. We then created a number of rules to make sure that these new depth charts were generated in a reasonable way, at least to the greatest extent possible.

Let’s use the 2025 Phillies as an example. As of the time of our run on April 7, we projected the Phillies for a .545 winning percentage against league average opposition. That projection comes from allocating playing time to each Phillies player according to our depth charts, using blended projections from ZiPS and Steamer to estimate the talent level of those players, and then plugging those projections into the BaseRuns formula to estimate runs scored and runs allowed. But those projections have an obvious weakness: they’re static. Read the rest of this entry »

I thought I had it sorted out; it took one batted ball to convince me otherwise. It was the bottom of the seventh inning during the Rays’ first game in their new George M. Steinbrenner (GMS) Field digs. Jonathan Aranda worked his way into a 2-0 count against Rockies reliever Tyler Kinley. With runners on second and third, one out, and the Rays down two runs, Kinley hung a slider. Aranda uncorked his A swing, launching the ball deep to right field. Off the bat, I thought it looked way gone. It didn’t even go 300 feet:

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