WPA
Description:
Most sabermetric statistics are context neutral, meaning they disregard the fact that during the course of a game some plays are more crucial than others. While wOBA rates all home runs as equal, we know intuitively that a home run in the third inning of a blowout is less important than a home run in the bottom of the ninth inning of a close game. Win Probability Added (WPA) captures this difference by measuring how individual players affect their team’s win expectancy on a play-by-play basis.
For example, if we were to take a specific game, say the Rays have a 45% chance of winning before Ben Zobrist comes to the plate. During his at bat, Zobrist hits a home run and the Rays’ win expectancy jumps to 75%. That difference in win expectancy (in decimal form, so +.30) from the beginning of the play to the end is Ben Zobrist’s WPA. If Zobrist strikes out during his next at bat and lowers his team’s win expectancy by 5%, his overall WPA for the game would be +.25.
Context:
Technically, WPA values for events could range anywhere from about 2% (.02 WPA) to 95% (.95 WPA), although the extreme swings in WPA are not terribly common. For an idea of how events typically influence WPA, here are the ten largest WPA swings of the Yankees’ 2009 postseason (brought to you by River Ave Blues):
Cumulative, season-long WPA is not predictive and should not be used to make projections about a player’s talent, but it is a good describer of what has happened. Here are some cumulative WPA values from 2010:
Things to Remember:
- WPA is not highly predictive. It should not be used for player analysis, but to tell who helped their team the most during the course of a game. A fun way to think of WPA is as Big League Stew a storytelling statistic. It tells us what the big moments of the game were and who the big players were.
- WPA is a counting statistic, like RBIs and HRs, meaning that players with more playing time will be able to accrue more.
- 1 WPA equals one win, since 1 WPA equals 100% of win expectancy.
Links for Further Reading:
Intro to WPA – Big League Stew
The One About Win Probability – Hardball Times
WPA is… WPA is not… – The Book Blog
What WPA Can Tell Us About Players – Hardball Times
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