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The New Science Of Hitting

In broadcast booths, on scoreboards and on Twitter, there’s a wave of new information sweeping across Major League Baseball. It tells us that Carter Capps’s leaping delivery increases his effective velocity and that Giancarlo Stanton hits the ball really hard. (OK, so we didn’t need Statcast to know that.) Those numbers are all courtesy of MLB’s Statcast system — which uses an array of radar equipment and high-resolution cameras to track every object and person on the baseball field — and they’re finally being released to the public this season.

Although MLB has made only a limited portion of that information available,1 Statcast’s new metrics have enormous potential to change our understanding of baseball, telling us not only what happened, but also how it happened. On the other hand, they’re largely unfamiliar to fans used to thinking in terms of old-school metrics. So today, I’m going to dive into two of Statcast’s new statistics — launch angle and exit velocity, both of which long existed only in the dreams of sabermetricians — and explore what they can tell us about hitters.

Launch angle measures the vertical direction of the ball coming off the bat; a launch angle of zero degrees would be a flat line, with positive numbers indicating an upward ball flight and negative ones indicating a ball driven into the ground. Hitters with high launch angles tend to be sluggers who produce lots of fly balls (and, sometimes, pop-ups). Kris Bryant was one of the league leaders in launch angle in 2015, with an average angle of 19.2 degrees. Conversely, low launch angles tend to be the domain of quick, slap-hitting middle infielders who generate lots of ground balls. Dee Gordon, for example, was toward the low end of the spectrum with an average launch angle of 2.9 degrees.

Exit velocity, on the other hand, represents the speed at which a ball leaves the bat. Although it was unofficially available from some websites last year, MLB officially packaged and released exit velocity alongside launch angle on April 7, effectively giving us a detailed measure of how hard each ball was hit. Unsurprisingly, at the top of last season’s exit velocity leaderboards you’ll find the game’s greatest sluggers — such as Stanton (99.1 mph), Miguel Cabrera (95.1) and Jose Bautista (94.3) — pounding out average exit velocities well north of 90 miles per hour.

We learned last season that exit velocity alone was modestly useful, but it becomes exponentially more powerful when combined with launch angle data. These two numbers together can tell us a great deal about what’s likely to happen after bat meets ball.

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The relationship between a batted ball’s launch angle, exit velocity and linear weights scoring value (a measure of the runs a play adds relative to average) is complicated.2 The very best hitters in MLB tend to smack lots of balls with launch angles around 25 degrees and exit velocities above 90 miles per hour, corresponding to the area of the plot rich in such valuable plays as home runs and doubles. Worse hitters, by contrast, have a tendency to make contact at sharper angles, where positive run values are harder to come by. That’s because balls hit with extreme launch angles (positive or negative) usually find their way into fielders’ gloves as either pop-ups or groundouts, doomed to be outs no matter their exit velocities.

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Meanwhile, the success of a ball struck at a more intermediate angle is extremely sensitive to its exit velocity. For instance, at a launch angle of about 25 degrees,3 run values can vary sharply depending on how fast the ball leaves the bat. Low exit velocities tend to result in short-hoppers to the infielders, which are easy outs. But as batters hit the ball slightly harder, those liners get progressively stronger, eventually sailing over infielders’ heads for bloop singles. Then the run value drops again, as those line drives begin to travel within reach of the outfielders. At a certain point, though, run value skyrockets again as hard-hit balls become doubles and, eventually, home runs.

(The valley in the chart above, with bloop singles on one side and doubles on the other, has been dubbed the “doughnut hole” because it’s surrounded by combinations of exit velocity and launch angle that are more productive.)

Except for those few line drives just before the doughnut hole, more exit velocity is generally better for batters. Although exit velocity most directly affects slugging percentage — harder-hit balls go farther and turn into extra-base hits more often — it is also correlated with almost every other positive indicator of offensive performance. It even affects how pitchers approach a given hitter; it’s positively associated with walk rate, presumably because pitchers fear throwing balls in the zone to high-exit-velocity hitters.

There are many things we can’t know from these two metrics alone, of course. Anyone who’s ever seen Billy Hamilton leg out an infield single knows that speed plays a significant role in what happens after a ball is struck. In fact, we can see the importance of swift base running by looking at ground balls and grouping batters according to their speed score, a Bill James-designed composite of stolen bases, triples and runs scored. The higher the speed score, the faster the runner.

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For grounders,4 there’s a big difference in scoring value depending on how quick the batter is on the base paths. Exit velocity being equal, the fastest third of batters always produce more value on ground balls than the slowest third. And the difference is especially pronounced with high exit velocities: At 94 miles per hour, a grounder is a net negative play for a slow runner, just about neutral for an average runner, and positive for the fast group. Because of this difference, low exit angles are much more harmful for lumbering first basemen than for speedy shortstops. The effect of speed starts to fade only when launch angles exceed 10 degrees, as exit velocity begins to take over as the biggest determinant of a batted ball’s fate.

Even with these preliminary principles from Statcast, there’s much we still don’t know. The statistics have just been released, so sabermetricians haven’t had time to fully digest them, much less conduct full-scale analyses. But every time new data finds its way into baseball, it teaches us fresh lessons. In the years after PITCHf/x was installed in 2006, we learned about the importance of catcher framing, a skill that had been regarded as largely fictitious. Who’s to say what novel theories will be born from Statcast data, now that we finally have our hands on it?

Footnotes

  1. Perhaps because of technical issues.

  2. This chart plots launch angles above -50 degrees and batted ball velocities higher than 15 mph. There were very few outliers below these thresholds.

  3. Specifically, I selected all balls with launch angles of between 22 and 28 degrees, somewhere between a line drive and fly ball.

  4. Defined here as launch angles between -50 and 10 degrees. For this graph, we are also considering only players with at least 250 plate appearances.

Rob Arthur is FiveThirtyEight’s baseball columnist and also writes about crime.

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