Monday, June 19, 2017

Are some of today's baseballs twice as lively as others?

Over at The Ringer, Ben Lindbergh and Mitchel Lichtman (MGL) claim to have evidence of a juiced ball in MLB.

They got the evidence in the most direct way possible -- by obtaining actual balls, and having them tested. MLB sells some of their game-used balls directly to the public, with certificates of authenticity that include the date and play in which the ball was used. MGL bought 36 of those balls, and sent them to a lab for testing.

It never once occurred to me that you could do that ... so simple an idea, and so ingenious! Kudos to MGL. I wonder why mainstream sports journalists didn't think of it? It would be trivial for Sports Illustrated or ESPN to arrange for that.

Anyway ... it turned out that the 13 more recent balls -- the ones used in 2016 -- were indeed "juicier" than the 10 older balls used before the 2015 All-Star break. Differences in COR (Coefficient of Restitution, a measure of "bounciness"), seam height, and circumference were all in the expected "juicy" direction in favor of the newer baseballs. (The difference was statistically significant at 2.6 SD.)

The article says,

"While none of these attributes in isolation could explain the increase in home runs that we saw in the summer of 2015, in combination, they can."

If I read that right, it means the magnitude of the difference in the balls matches the magnitude of the increase in home runs. The sum of the three differences translated to the equivalent of 7.1 feet in fly ball distance.

The authors posted the results of the lab tests, for each of the 36 balls in the study; you can find their spreadsheet here.


One thing I noticed: there sure is a lot of variation between balls, even within the same era, even used on the same day. Consider, for instance, the balls marked "MSCC0041" and "MSCC0043," both used on June 15, 2016.

The "43" ball had a COR of .497, compared to .486 for the "41" ball. That's a difference of 8 feet (I extrapolated from the chart in the article).

The "43" ball had a seam height of .032 inches, versus .046 for the other ball. That's a difference of *17 feet*.

The "43" ball had a circumference of 9.06 inches, compared to 9.08. That's another 0.5 feet.

Add those up, and you get that one ball, used the same day as another, was twenty-five feet livelier

If 7.1 feet (what MGL observed between seasons) is worth, say, 30 percent more home runs, then the 25 foot difference means the "43" ball is worth DOUBLE the home runs of the "41" ball. And that's for two balls that look identical, feel identical, and were used in MLB game play on exactly the same day.


That 25-foot difference is bigger than typical, because I chose a relative outlier for the example. But the average difference is still pretty significant. Even within eras, the SD of difference between balls (adding up the three factors) is 7 or 8 feet.

Which means, if you take two random balls used on the same day in MLB, on average, one of them is *40 percent likelier* to be hit for a home run.

Of course, you don't know which one. If it were possible to somehow figure it out in real time during a game, what would that mean for strategy?


UPDATE: thinking further ... could it just be that the lab tests aren't that precise, and the observed differences between same-era balls are mostly random error? 

That would explain the unintuitive result that balls vary so hugely, and it would still preserve the observation that the eras are different.

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At Tuesday, June 20, 2017 11:59:00 PM, Anonymous Kevin said...

~25ft range of variation between MLB balls is consistent with what Alan Nathan found in his own testing

At Friday, June 23, 2017 11:08:00 AM, Blogger Zach said...

My prior would be that the testing is likely more precise than the controls to insure input material consistency. I'd expect they have some quality control over the inputs but I'd be surprised if they have much of a precise metric regarding these coefficients.


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