Astrophysicist and sports data scientist Meredith Wills talks about why a subtle change in Major League baseballs may be behind the jump in home runs after 2014.
Steve Mirsky: Welcome to Scientific American’s Science Talk, posted on September 30th, 2018. I’m Steve Mirsky. On this episode:
Meredith Wills: A good way to change the drag is to change the spherical symmetry and basically a round revolve is gonna be able to have less drag and therefore travel further so my thought was that the seams are basically the weak points.
Mirsky: That’s Meredith Wills (no relation to Maury). She has a doctorate in astrophysics and has authored studies that appeared in the Astrophysical Journal and in the journal Solar Physics. More recently, she’s been studying the game of baseball, as a sports data scientist. On September 19th she published an article in the online publication The Athletic about her studies of the baseball itself. In an attempt to figure out why home runs have gone through the roof—from 4,186 home runs in 2014 to 6,105 in 2017. I spoke with Wills by phone.
Home runs are significantly up in Major League Baseball and a lot of actual scientists have been trying to figure out what’s going on there so what was it that you were looking for and what did you find out?
Wills: Well, I guess I should point out that this was a question that, in 2017 when it was going on, a lot of people were approaching from a lot of different ways. Some people thought it might be the baseball. Some people thought it might have to do with something called launch angle, just the angle at which the ball is coming off the bat and that there were players who seemed to be altering it and so, therefore, you might get home runs. There were a lot of different approaches and ultimately MLB commissioned a Home run Committee, which as you say were – the chair is – Dr. Alan Nathan who is a Professor Emeritus at of Physics at Illinois. They had engineers.
They had mathematicians. They had, you know, it was a very stem-heavy committee and they went through and they did a lot of lab testing and simulations and comparison of data and what they found was that there seemed to be one statistically-significant change, which was basically that the baseball itself starting during 2015 but definitely after 2015 had less aerodynamic drag. What they did not find was why they had less aerodynamic drag. Clearly the change was the baseball but the committee itself didn’t find a difference in the baseball. I had been doing my research thinking okay, this might have something to do with the baseball and it turns out there’s good precedent for that because when changes have been made to the baseball in the past it actually has changed how the ball moves.
The best example of that would be the transition from what’s called “the dead-ball era” to “the live-ball era” (you have different eras in baseball) actually corresponded to the change in the source of the wool that was used for the yarn inside the baseball. The inside of a baseball is largely yarn, wrapped around a core and before, during the dead-ball era, all of the yarn was from American sheep but because of World War I, we ran out of wool and we couldn’t make baseballs using American wool so we started importing it from Australia. The Australian yarn turned out to behave differently and suddenly you get what’s called the live-ball era and people hitting home runs and Babe Ruth and that sort of thing so the actual ball was different because of the wool being different
Mirsky: American baseballs had Australian wool in them?
Mirsky: This is sacrilege.
Wills: Well you know they do … first of all they do play baseball in Australia so there are Australians out there who might take issue but quite good baseball, in fact. So back to the ball, I thought okay, maybe there’s something different about the interior of the ball that was having a similar effect and so what I did was I very, very systematically took apart two samples of baseballs, one that I knew was from 2014 and another set that I knew was from 2016 and 2017. Alan Nathan (back to him) had already stabled that the change had occurred over 2015 so I could treat 2016 and ’17 as a single sample set and this was literally the unlacing the red laces on the baseball and I measured, you know, all sorts of different things: Circumference and mass and you know different factors on the leather, different factors on the yarn inside and there’s three different kinds of yarn. I ended with 16 independent variables and I love how science works because this was literally an accidental variable. My 16th variable I didn’t even think of until I started doing a different measurement and realized hey, this looks wrong, had to do with the thickness of the laces and thought okay, the thickness looks different between the two populations and, again, it just had to do with the way I was doing a particular measurement and so I went in and I measured the thickness. And there’s a way you can do it where you just you know basically wrap the thread around a dowel and you can figure out how many what’s called wraps-per-inch or in this case I used centimeters because it’s so thin and I found a statistically-significant difference in the thickness of the laces between 2014 and 2016 and ’17 and it’s 9 percent.
And it was a noticeably statistically significant difference. They’re thicker now so they’re 9 percent thicker now than they were before 2015. And so this turned out to be a really big deal because the only thing that the Home run Committee had found was that the difference was the ball but they couldn’t figure out what was different about the ball. And in fact their particular study [laughs] got published about 3 days after I found this result so I spent a couple days sort of you know metaphorically jumping up-and-down saying, “I know the answer, I know the [laughs] answer” and ended up, you know, this written up as an article for The Athletic and published 2 weeks later saying, “Here’s what the difference actually is with the ball, there really is one.” Now for most people it seems like thicker laces shouldn’t make the ball have less drag because they tend to think that the laces are the same thing as the seams and thicker laces, they would think, gives you higher seams, which should give you more drag, not less.
That’s actually a fallacy. It turns out that lace thickness and seam height are 2 different things. I postulated that having thicker laces because they have greater tensile strength they might actually be somehow keeping the ball more spherical than if the laces were thinner and so I did a follow-up study where, instead of using all the baseballs that I had taken apart because obviously I, you know, you can have your cake or you can eat it, [laughs] basically. And so I got a new set of baseballs, one of which was, again, pre-2015. The other was post-2015. It was a larger sample set and I hypothesized that if there was an aberration in the spherical symmetry of the ball because a good way to change the drag is to change the spherical symmetry, you know? Basically a rounder ball is gonna be able to have less drag and therefore travel further so my thought was that the seams are basically the weak point. If you’re gonna have any deviation from spherical symmetry it’s probably gonna be nearer the seams and, on top of that, I thought, okay, thicker laces are more likely to give seams that stay more intact therefore you could get a more spherical ball.
So just taking essentially a set of calipers [laughs] I looked at what I called an average diameter, which was essentially points on the ball, and the same points on every one of my baseballs, away from the seams, and then I also looked at diameters, what I called adjacent to the seams so not the seams themselves because they’re too high but, like, say, 2 millimeters off, and there’s a couple places on the ball where you can measure the diameter and you get a seam adjacent on both sides and what I discovered was that for all, I had 20 balls from before 2015 and what I discovered was that for all 20 of those baseballs they were bulging at the seams. That seam-adjacent diameter was larger than the average diameter, every single baseball from pre-2015 all of which have thinner laces. When you look at the newer baseballs, which have thicker laces, two thirds of them did show some bulging near the seams but not nearly as much and one third of them actually showed essentially un-bulging, as it were, and it was actually narrower near the seams so there wasn’t anything systemic in the newer balls and so what it looks like to me is that, yeah, the issue is the seams and the thicker laces are somehow keeping the seams more intact therefore the ball is more spherical and therefore you have less drag, therefore you have more home runs. So it is the laces.
Mirsky: You have a unique background to do this work. You have a doctorate in Astrophysics. You do statistical analytics on baseball and you’re also an enthusiastic knitter and so you tweeted when you published this in The Athletic the other day, you tweeted, “I just realized that reaching this conclusion required a detailed knowledge of physics, baseball and fiber arts. Who would’ve thought those skill sets would ever come together?”
Wills: I was … I will admit that thinking about it I was pretty surprised. Part of the detailed knowledge of fiber arts comes from the fact that by knowing how different, different fibers, say, cotton, wool, that’s what I mean by different fibers, you know? Polyester counts I guess although we’re not discussing that here. I know how they react under different conditions because I do a lot of knitting design. I’ve done costume design so I’ve worked, you know, when I say “fiber arts,” I mean all sorts of different things. Basically if it involves needles and thread I’ve … or yarn, I’ve done it and cotton reacts in a very particular way when it gets wet, which a lot of people, particularly because we have dryers nowadays don’t think about but if you have wet cotton and you distort it somehow and then you let it air dry it actually will retain its shape so we’ve all had the experience or most of us have had the experience where, let’s say, you spill coffee on your shirt during the day and you obviously don’t want the coffee stain to stay there so you go into the bathroom and you scrub out your t-shirt in the sink and the divot stays there, you know?
Until you actually take off the shirt and wash it, you’ve got a divot there for the rest of the day. The reason the divot stays there is because you’ve distorted the cotton and then you allowed it to air dry, and it takes throwing it back in the dyer to get rid of the divot. The way that the baseballs are made the leather itself -- and it makes sense because obviously the leather has to you know conform to the ball, it’s gotta get round -- gets moistened and then they pulled the red laces, which happened to be made of cotton, through the wet leather so you end up with wet, cotton laces. What they then do and obviously they’re pulled super-tight, you know? If you’ve ever tried to pull the laces out of a baseball or even just tried to pull ‘em up with your fingernail they’re incredibly tight.
On top of that there is actually a pressure process that goes into once the balls are done to try to flatten the seams so first you have them pulled tight, then you have those seams put under pressure, while they’re wet, and then you allow them to dry, all of which is effectively going to stretch out the cotton and for thinner laces, that cotton is gonna stretch more, which means you’re gonna have a weaker seam that’s more likely to bulge. So that’s where the fiber-arts knowledge comes into play. I’m pretty sure … let’s put it this way the Home run Committee would probably have eventually figures out that the distortion was the seams. The fact that I had the knowledge of what cotton would do, under those circumstances, meant that I was able to get to get the conclusion much faster.
Mirsky: That’s great. I just wanna let people know the names of some of your other publications from earlier in your career: “Statistical Study of Coronal Mass Ejections with and without Distinct Low Coronal Signatures.” That’s a good example. Let me find another one here.
Wills: There’s a few on something called EIT waves, which are basically these large-scale propagating fronts within the corona that I … that was sort of my specialty but they were named after a particular instrument. I think one of the early terms that was used for them was solar tsunamis, although I believe that’s now used for something else so if you’ve heard the term solar tsunami that would be what were originally referred to as solar tsunamis were those and so I did a lot of the initial work on that. I was the first one for example to come up with an automated detection method for actually tracking these events because they’re very dim, the signals-to-noise is terrible and for a long time people would only track them by eye because they thought there was no way that you could deal with the signal-to-noise such that you could actually track the front using your computer and I did it and so that was kind of you know one of my big contributions within solar physics. That also turns out to have carried over well into baseball because now everything is tracking, you know? You’re tracking the pitch.
You’re tracking the ball when it’s hit. Now we’re actually tracking the players, which I think is really [laughs] awesome and that is much more like what I used to do so I’m particularly interested in player tracking partly because it’s similar and it’s a complex problem, which I like, but also because I’m a total defense geek and so it means that they come together, which was kind of fun.
Mirsky: So player tracking, you’re talking about things like route efficiencies?
Wills: Route efficiencies would be one or tracking to see, for instance, a jump when a guy steals a base you know what his ump is like. Player tracking is actually one that is still kind of nascent because, like, route efficiency is a useful one in a way but it’s not as useful as people think. Route efficiency is really cool if you have to sprint a really long distance to get the ball. On the other hand if you’re playing a normal fly ball, say, having the most efficient route isn’t necessarily a good idea because you won’t be set up, say, to throw the ball to second base to cut off the runner on first. So there are there’s a lot more subtlety to it.
One of the things that you can pull out of player tracking, which you know again it’s nascent, it’s still being done, are things like looking at how players set up for, say, that throw or set up for a cut off man or even things like getting multi-player kind of statistics. So how good your first baseman is based on how good the people around him are. Is it that he happens to play the position really well defensively or is it that you have a really, really good infield, meaning you can get away with having a first baseman who can’t field the ball or can’t catch the ball, I should say?
Mirsky: They’re not giving him a lot of bad throws that he has to scoop outta the dirt.
Wills: Right and so you got someone like you know I could throw out names like Todd Helton or Mark Teixeira or whatever people who were incredibly good first basemen who could you know do a half a split or a full split off the bag and feel the bag bounce which meant you could surround them with people who aren’t necessarily as good and they can compensate, you know? There’s also for instance you can look at efficiency of players running the bases or even just how a player … it depends on the type of hit but how do they accelerate of into first base? There are some guys where their jump is immediate. Ichiro Suzuki was, like, the perfect example of that. I could never figure out how he could be two steps out of the box when he the ball but somehow he always seemed to do it or you have guys who are almost still ramping up their speed when they get to first so they don’t hit as many singles but once they get going it’s easy for them to hit a double, if that makes sense, you know, because they’re not sprinters in the same but they absolutely can pick up their speed or even just the route that you take around, you know? If you’re running a straight line to first base that’s really useful for a single but if you wanna double, you wanna come in at curve because otherwise you’re actually not taking the most efficient route to get to second base if you, essentially, put a right angle from first to second so those are the kinda things that you would track, looking at the players themselves.
Mirsky: Right and you’re … the name of that article that you published in 2008 was “Tracking Large-Scale Propagating Coronal Wave Fronts (EIT Waves) using Automated Methods” and that has come back to actually be something that your background doing that is helpful now for doing these kinds of tracking of players. It’s really amazing.
Wills: Yep and it turns out that the data are less dissimilar than I would’ve expected, which is kinda fun so it’s really cool to be to take one thing and carry it over into another.
Mirsky: And now more with Meredith Wills. We’ve heard a lot … if you’re a baseball fan, you’ve heard a lot about exit velocity and launch angle and that players are trying to hit the ball up-in-the-air over the last few years and so most people probably would’ve assumed that the launch angle had increased but the actual analysis showed that there was no difference in average launch angle between 2014 and 2016 and ’17 when there was a big difference in home run rates.
Wills: The important thing to realize is that the home run rates went up globally, you know? It wasn’t just that you had a few players who suddenly went from hitting 30 home runs to hitting 55 home runs or something like that, what you had is you had guys who normally hit 10 home runs who were suddenly hitting 15 and everybody was doing it so that’s why the numbers went up like that because there was some systematic change throughout the game. There are absolutely players who have changed the way they hit the ball and change their launch angle. A great example would be a couple years ago Daniel Murphy when he was with The Mets during the NLCS and he hit, what, 6 home runs in the 6 games?
Mirsky: Yeah, he hit a home run every day.
Wills: Every game against The Cubs and it was just amazing. I remember that specifically because apparently the goat from the curse was named Murphy and so I was like, okay, well, maybe it’s not a goat curse, maybe it’s a Murphy curse.
Wills: You know but anyway the … at the time everybody thought it was an aberration but by the next year he showed very clearly he had become power hitter and then explained in interviews that before the postseason, he had actually started working on his swing mechanics to change his launch angle. The thing is though not everybody has done that and even if they’ve tried, not everybody has been successful. I mean there’re some guys who can change their swing mechanics and there’re some guys where changing their swing mechanics might just mess with their swing and they probably shouldn’t. So in a way looking at something like launch angle you can see it with individual players but to have a global change it would require everybody suddenly deciding to do this and everybody doing it successfully so I think that was … again, it’s a good thing to look into and the Home run Committee looked into all of this, actually. They had an incredibly … it’s an 80-page report. It’s very thorough. It explains a lot and they look at pretty much everting that was postulated during 2017 as far as what, you know?
They looked at essentially, you know, climate change as maybe having an effect with temperatures increasing and therefor the ball might travel further. They looked at launch angle. They looked at … I think they actually looked at strategy and when you took out all the variables basically what they found was that the ball was travelling further. For a given exist velocity and a given launch angle the ball was traveling further, regardless.
Mirsky: Yeah and there was an observed decrease in drag coefficient that they found as well.
Wills: Yes so when they removed all the rest of the variables that was the only one that stood out as statically significant. There were other changes but not ones … only things that might show up as trends and nothing that showed up as meaningful. What they were looking for was a genuine change and that was the only statistically significant change was the drag.
Mirsky: And that’s what your stitches study tries to account for and may account for.
Wills: Yeah one thing I guess I should point out is with the older balls that are bulging at the seams, baseballs are all made-by-hand and so there does tend to be a fair bit of variation from ball-to-ball so I don’t know if we will … it would take a huge population probably to find a statistically significant difference as far as that bulging effect so, in my case, it’s more that the trend is there and that the fact that all of the balls show bulging is telling but for the newer results it’s not statistically significant so please, you know, if you’re really, really determined to hang your hat on it, that’s yours, that’s not mine, [laughs] you know?
Mirsky: You are proposing a viable hypothesis with this study for the findings of the committee studying home run rates.
Wills: That is an excellent way to put it.
Mirsky: Okay good. I just want people to know the report. It’s called “The Report of the Committee Studying Home Run Rates in Major League Baseball” and that’s dated May 24th, 2018 and that’s available free, for nothing, on the web and your article in The Athletic is titled “Studying the baseball to find the ‘how’ of the home run surge” and The Athletic is by subscription. I am fortunate to be a subscriber.
Wills: I should also point out there was a previous. The previous article was published on June 6th and the title is something like “How one tiny change in the baseball may have led to the home run surge and the rise in pitcher blisters,” which is a whole different thing. It turns out thicker laces seem to be causing pitcher blisters. We can leave it at that. That’s the next study.
Mirsky: This was great. I just find this stuff endlessly fascinating as a science-interested person and a baseball fanatic; I just love all this stuff.
Wills: It is so nice to hear you say that you’re a baseball fanatic, having worked with scientists with so much of my life there just aren’t enough of us. There should be more.
Mirsky: Going into the last day of the season, we had 5,550 homers in 2018. Not as many as last year’s 6,105 but still way up from the 4,186 in 2014.
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