Thursday, May 8, 2008

Yaye Probability Theory

[I realize that it's generally spelled "yay". For some reason that bothers me, and so I add an "e" to the end of it. I've been doing it for as long as I remember, and I will continue doing it for the foreseeable future. Sorry if this bothers anyone.]

Win Probability is an interesting stat. You can read about it here, but basically it's the increase/decrease in the probability a team is going to win the game based on a given event. An example:
"Bottom of the ninth, score tied, runner on first, no one out... Let's say our batter in the bottom of the ninth hits a single to put runners on first and third with no outs. This increases the Win Probability from 71% to 87%, for a gain of 16%. So, in a WPA system you credit the batter +.16 and debit the pitcher/fielder -.16. If you add up every positive and negative event from the beginning to the end of a game, you wind up with a total for the winning team of .5, and a total for the losing team of -.5. And the player with the most points will have contributed the most to his team's win."
The probabilities are found by looking at historical data to see how often teams actually won when in the situations.

The Hardball Times has the total WP added for each team, broken down by batting, starting pitching, and relief pitching. Here are ten things that I found interesting:

1) Offense really is down in the AL, as only two teams (LAA and BOS) have positive WPA for batters. The Angels are actually the only team to have a positive WPA in all three aspects.

2) The Indians, despite poor starts for their two best pitchers, have the highest WPA for starters in the league.

3) The A's have outscored their opponent by a ridiculous 42 runs (Boston is next closest at +26) largely on the strength of a fantastic bullpen (3 WPA).

4) Texas has been outscored by 37 runs (KC is next at 27) and are the only team to have a negative WPA in every category.

5) Seattle has a strong rotation (1.12 WPA) but a bad offense (-2.49) and a bad pen (-2.13) have them at a league worst -3.5 WPA overall.

6) The O's offense has been bad (-1.68, which is 8th in the league) but the pitching (0.11 starters and 0.58 relievers) has been OK. Overall, their at -1 win added, which goes along nicely with their 16-18 record.

7) Over in the NL, the Padres offense has been absolutely horrible (-7.35 WPA) leading to an MLB worst -5 WPA overall.

8) On the other end of the spectrum , Arizona has had the best offense (3.27 WPA) and positive contributions from the rotation and bullpen to sit at 5 WPA overall (tops in the majors).

8) Atlanta is using a surprisingly effective rotation (2.88 WPA) to make up for a surprisingly ineffective offense (-1.81 WPA).

9) Cincinnati and Washington are in the negatives in all three categories, though the defending NL champion Rockies are worse than either overall at -4 WPA.

10) San Fransisco does have an awful offense (-4.41 WPA) but their starting rotation hasn't been that great overall either (-0.27 WPA).

I stopped at ten because it is a nice round number and I didn't feel like writing anymore about WPA at this point. I'll check back on this stuff in a while.

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