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February 20, 2014

Overthinking It

Last Season in Selective Aggression

by Ben Lindbergh


Last April, Astros manager Bo Porter was asked about the secret behind his team’s hot hitting during a three-game winning streak. “Selective aggression,” he said. And last October, before NLCS Game Six, Pete Kozma was asked to explain the Cardinals’ season-long offensive success. (All of the good hitters must have been busy.) His response: “selective aggression.” Spring or fall, worst team or best, the answer was often the same. Selective aggression was what every offense strove for.

So what does it mean?

“To me, [it means] you swing at a high percentage of strikes but rarely chase out of the zone,” says one pro scout. “A selectively aggressive hitter is a dude who won't often chase out of the zone but isn't afraid to hack at the 0-0 get-me-over fastball.”

It’s hard not to like the sound of that, which is why “selective aggression” has become a cliché. So which hitters best embody the selective/aggressive approach, and which haven’t gotten the hang of it?

Just for fun, I decided to look for the most and least selectively aggressive hitters in the majors last season by comparing each player’s in-zone and out-of-zone swing rates from our Batter Plate Discipline report (derived from PITCHf/x) and sorting for the greatest discrepancies. And I really do mean just for fun, since this isn’t much use from an analytical perspective. The ratio of Z-Swing Rate to O-Swing rate is only weakly correlated with offensive production, as measured by TAv (r=-.19, from 2008-13), and it’s hardly any more closely correlated than O-Swing rate alone (r=-.16). Hitters who are selectively aggressive tend to be better than those who aren’t, but the connection is tenuous, maybe because swing rates alone tell us little about how often the swings make contact, or how high-quality that contact is. You or I might be selectively aggressive, but we would still be bad at major league baseball.

There are other problems with Z-Swing:O-Swing ratio as a stat, namely that we’re smushing together two different denominators (pitches in the zone and pitches out of the zone) and we’re assuming that all pitches in the zone should be swung at, which obviously isn’t the case. (On a 3-0 count, for instance, it makes sense to take the strike on the corner and hope the pitcher comes in on 3-1.) But because this exercise is silly in concept, it bothers me less that it’s somewhat silly in execution. Sometimes silliness is what we want.

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