Red Sox WPA through 4/30/06
Since it's the biggest positive WPA factor in today's game, this is a good time to provide a little more explanation on the "Opponents' Errors" category. In each game, the WPA for each team must sum to either 0.5 or -0.5 (depending on whether the team wins or loses). In most at-bats, the batter gets credit (either positive or negative) for the change in the win probability during his at-bat, and the pitcher gets equivalent credit in the other direction. Each at-bat is therefore a zero-sum transaction - if the batter gains, the pitcher loses, and vice versa. However, in the case of an error during a Sox at-bat, the batter does not usually deserve credit for reaching base. The error means that he should have been out, but reached only because of the error.
In a situation like this, when calculating the WPA, I treat the at-bat according to what would have happened had it not been for the error (which usually results in a negative WPA for the batter, and a positive WPA for the pitcher). Then I enter another transaction that accounts for the difference between what should have happened and what actually happened. The positive WPA for this transaction is credited to a catch-all category called "Oppenents' Errors", and the negative WPA is credited to the fielder on the opposing team who made the error.
The result of all this bookkeeping is that the category called "Opponents' Errors" ends up measuring the extent to which the Sox have benefited from errors made by the opposing team. Today, this was a particulary large category. In the seventh, Varitek reached on Burroughs' throwing error. If he had been out, Varitek would have received -0.035 WPA. Instead, reaching first in that situation (down by 2 in the 7th inning) results in a WPA increase of 0.056. The difference between the result that would have happened had the error not occurred and the actual result, which is 0.091, gets put into the "Opponents' Errors" category. Similarly, in the ninth, Harris stole second and advanced to third on the bad throw from Hall. Getting the tying run to second in this situation is quite important, and Harris gets 0.052 WPA for the steal itself. But the advance to third is even more important (there was only one out at that point), and worth an additional 0.097 WPA, and this 0.097 gets credited to the "Opponents' Errors" category.
Confused yet? It's really just a bookkeeping transaction to make sure all the numbers add up, but it's interesting (and depressing) to see that at this point in the season, the team has received substantially more benefit from their opponents' errors than they have from the combined performance of the whole offense.
Individual Player WPA Contributions
Red Sox 4, Devil Rays 5
Through Sunday, 4/30/06
Full-Season Category WPA Contributions
Through Sunday, 4/30/06