I've finally taken a crack at something I've wondered about for awhile, and I'm not sure what it means, if anything. My thinking goes like this.
There are two things that a hitter must do to hit a pitch:
- Recognize the pitch.
- Correctly estimate its movement and velocity and then get the bat to the right place at the right time.
But here's the interesting part. Changeups with a ton of movement fall off the table down and away from opposite-handed batters and create ground-balls on account of their downward movement. And when the batter is fooled into thinking its a fastball, that means that he thinks the ball will be in the air for less time than it actually is, so gravity acts on it more than he expects, and it also creates ground-balls.
Now here's my idea. When a hitter thinks that a cahngeup is a fastball, he swings too soon, and he's out in front. That means that for any given pitcher, if he gets considerably more ground balls on changeups that are pulled than on changeups that are hit the other way, that means that his changeup success is more due to arm action and deception than it is to movement.
Using Baseball Savant, I pulled the records for every right-handed pitcher who's had 50+ changeups put in play by left-handed hitters both to the pull side (first base, second base, and right field) and the other way (third base, shortstop, and right field). I then calculated and compared the GB% of those changeups put in play to each side of the field.
Take a look at the Google Doc.
Because I limited it to pitchers with many changeups put in play, it's a group who all have pretty good changeups. I'm not sure what this means, or if it means anything at all, but Alex Cobb is on the extreme end of the list.