It's something of a well-known tale by now. The Mets, after years of trying to decide between lefty first basemen Lucas Duda and Ike Davis, settled upon Duda early in the 2014 season. Why? In large part because of Duda's superior exit velocity, which is to say that batted balls came off of his bat harder. It ended up being a wise choice: Duda has hit 57 homers over the past two seasons while Davis bounced from Pittsburgh to Oakland and is currently a free agent after being non-tendered by the A's last month.
The Mets, and most other teams, have been able to measure exit velocity for several years, and they clearly place value on it. But as far as the public goes, the 2015 debut of Statcast™ was the first time that such measurements made it out into the wild. Having a full season of data under our belts, and having seen how useful it can be in tracking hitters' health, how much does exit velocity actually matter?
The answer: a lot, though it's hardly the only thing that goes into making a successful hitter. We took the hitters who qualified for the batting title and mapped their average exit velocities to their Weighted On Base Average (wOBA), an advanced stat quite similar to traditional on-base percentage but which gives additional value to extra-base hits over singles and walks, rather than treating each time on base equally. For reference, the 2015 average wOBA for non-pitchers was .318.
The correlation, as it turns out, is a good one. This shouldn't be surprising; hitting a ball hard is a skill, and being able to do so consistently makes it more likely you'll be productive. (Pitchers issuing more free passes while attempting to work around these stars doesn't hurt, either.) Names in the top 10 qualified hitters on the exit velocity leaderboard include Miguel Cabrera, David Ortiz, Jose Bautista, Mike Trout, Josh Donaldson, Nelson Cruz and Paul Goldschmidt, all celebrated sluggers, and so that sounds like a good start.
Yet you'll notice it's not a perfect correlation, with a fair amount of variance on either side of the trend line, and that's also to be expected, because the beautiful frustration of baseball is that it doesn't always reward what would otherwise be considered peak performance. For example, Carlos Gonzalez and Hanley Ramirez each had outs on balls that were hit at 117 mph right at fielders, while a number of players found singles on well-placed balls below 50 mph.
How much contact you make -- can't get an exit velocity reading without it -- and walks drawn are obvious non-exit-velocity inputs to production, as well as the angle of the batted ball. Exit velocity can tell a lot about the likelihood of a hit, but as Gonzalez and Ramirez found out, the direction has a lot to say about it too.
It's that wiggle room that can allow for some interesting outliers, like Washington teammates Bryce Harper and Wilson Ramos. Harper just had one of the most legendary seasons in history, while Ramos disappointed, putting up the worst season of his career. Each had similar exit velocities (91.5 mph for Harper, 91.1 for Ramos) and each had a 20-percent strikeout rate. So how did Harper's season end up so much better?
Well, Harper's walk rate being nearly five times higher than Ramos' is a big part of it, and so is the fact that he's much faster -- as a runner from the batter's box, Harper reached a top speed of 19 or more mph 81 times, compared to Ramos' five. But the real difference is in the fact that Ramos pounded everything into the ground, with a 4.2-degree launch angle (and a 55.5 percent ground-ball rate, the eighth highest in baseball), while Harper's 12.3-degree launch angle gave him far more line drives and fly balls, which often left the park. Exit velocity is a very good start, but it's a partial picture until it's married to launch angle.
What if we mapped exit velocity to traditional batting average instead? That eliminates walks entirely and focuses more on batted balls, but the results are without any correlation at all:
Why? Because batting average's main flaws are that it ignores walks and treats all hits the same, giving no extra credit to a homer over a single, and we know that higher exit velocities can lead to more extra-base hits. Soft-hitting speed types, like Dee Gordon and Billy Burns, the two lowest exit velocities on the chart, have high batting averages because they make contact and beat out balls, but less impressive wOBA marks because they have little power and rarely walk.
So is exit velocity for a hitter meaningful? It sure seems like it is, if you consider overall offensive production, and this is the closest equivalent we have to a batting version of a radar gun for pitchers. We'll have to see how well the year-to-year correlation holds up after another season of data. But just as some pitchers can do well with an 88-mph fastball while others struggle throwing 99, velocity alone doesn't lead to success. There's a lot of ways to be a valuable hitter, and regularly putting hard contact on the ball is a big one. We've always known that; now, we can measure it. It seems like it's worth our while to do so.