Tuesday, July 25, 2006

Ball trajectories: do they increase the reliability of offensive stats?

In their JQAS article (mentioned below) “Who Controls the Plate? Isolating the Pitcher/Batter Subgame,” the authors adjust every player's statistics by where he hits the ball. For instance, if a player hit five line drives to an area of the outfield where, historically, 80% of balls have been hits, the player would be credited with the equivalent of 4-for-5, regardless of the actual results.

(Recently, ProTrade used the same idea to figure out
which players and teams were having lucky years.)

The idea, I guess, is that this gives you a more realistic picture of the player’s performance than counting the actual results. After all, if two players on two different teams hit exactly the same ball, but one was caught by team A’s gold-glove right fielder, while the other was missed by team B’s slow-footed old guy, why not treat them equally? After all, it’s not B’s ability that got him the hit. He was just lucky to be facing an inferior defense that game.

But there’s another side to it. For some hits (line drives specifically), the difference between the hit areas and the out areas is quite small -- a few feet.

Suppose again that the two players hit the ball exactly the same way, but this time the two fielders were playing in slightly different positions. Under the “credit them by where the ball was hit” method, they both get treated the same. But what if one saw where the fielder was standing, and deliberately hit a line drive in front of him to land for a hit? In that case, they shouldn’t be treated the same, because, even though they did the same thing, player A did it because it was the better thing to do, while player B failed to do the better thing and hit the ball a bit differently.

And, of course, there are many cases of the defense repositioning itself for a pull hitter. For those hitters, the “go by where the ball was hit” method won’t work. With Barry Bonds batting, the defense shifts from where the average player is likely to hit the ball, to where Barry Bonds is more likely to hit the ball. Bonds hits lots of balls that would drop in for hits if, say, Randy Winn hit them, but are easy outs with the defense shifted. The trajectory method would be completely invalid for a player like Bonds, unless you had a separate set of data for a defense in the Bonds configuration.

Given all that, I’m thinking it might actually be less accurate to go by where the ball was hit, and you’re better off just recording whether it was a hit or not. The simpler way, sure, you’re going to credit players with hits and outs they didn’t deserve. But those come randomly, and there are ways to handle random errors. The location way, the results are biased for many players -- and that can ruin a lot of potential studies. (The JQAS study is not really affected because it doesn't depend on the numbers of any specific players.)

But I don't really know which is better. You could find out buy running a study -- just see if players' trajectory-adjusted stats tend to predict future batting average better than batting average itself.


At Tuesday, August 01, 2006 5:10:00 PM, Anonymous Anonymous said...

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At Monday, August 07, 2006 2:27:00 AM, Anonymous Anonymous said...

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