THE BOOK--Playing The Percentages In Baseball

Wednesday, September 22, 2010

Park component numbers by adjusting for identity of hitters and pitchers

Just last week, I was explaining how to do park adjustments, you should use the component lines of the players, and not presume that each player plays an equal number of games home and away.

Analysts privately calculate their own more specific park factors.  As I said, among other things, I look at who actually played in the parks, not presuming that CC pitched exactly one time in the 13 other AL parks and 13 times at Yankee Stadium.

Well, it looks like Colin is going to apply those kinds of park adjustments:

So for every player, in every park they played in, I put together a set of component batting lines--I call them batting lines, but I did them for pitchers as well (call that batting line against, if you prefer--pitchers as hitters were excluded). For a player’s home park, I adjusted the batting line based upon observed home field advantage over a five year period.

These were combined into what I called “road weight” batting lines for each park in which a player played. If the park under consideration was not a player’s home park, the road weight batting line was simply their batting line for that season. If it was their home park, their games played there were given 1/15th the weight and averaged with their road stats. All of these component batting lines were then regressed to the mean – “noisier” components were regressed more than more stable components.

Then, for each batter-pitcher matchup in that park, I calculated an expected set of probabilities using the odds ratio method. (When the home team batted, the component home field advantage was added to the component batting lines of both hitter and pitcher.)

I don’t agree with the need to do the 1/15th.  I understand why he would do that (it makes the thing stabilize faster).  But at the cost of losing nearly half your sample.

He also says:

What these park factors are good at is telling us, with a certain amount of uncertainty, how a player might have played in a different context. Which is what we’re interested in with park factors, isn’t it?

Well, not exactly.

To MGL, that is yes, exactly.  To people trying to assign past value, then it’s no, exactly. 
- MGL wants to know what Juan Pierre will hit in a neutral park, or his expected 162 park-games. 
- Most people want to know how Pierre’s past offense impacts his past team’s chance of winning.  Put someone who can’t take much advantage of Coors, while Dante Bichette takes huge advantage of Coors, and Bichette’s value starts to soar, relative to what it would have been otherwise.  LHH gets killed at 3Com, while Barry Bonds is unaffected: this gave Bonds much greater value.

To each his own.