First of all -- what is WPA?
Win Probability Added. As Hardball Times's Dave Studeman has written, it's gone by a lot of different names. "Player Win Averages," "Player Game Percentage," "Win Expectancy," "Player's Win Value," etc. But WPA is the the most common name for it these days.I received a spreadsheet for Win Probability from one of the guys at Lookout Landing a couple of years ago, but never found the time to incorporate it into the blog. Maybe I should look at doing that in 2010?
How they calculate WPA: Though the arithmetic can be a mother, Win Probability Added is one of the easiest of all the advanced stats to explain. Put simply, it's a measure of how much any game event contributes to the eventual outcome of the game, win or loss.
Or, as Fangraphs' David Appelman writes, "WPA is the difference in win expectancy (WE) between the start of the play and the end of the play." (For those of you who remember calculus, this means it's a marginal measure — it measures the difference in probability between two states. Feel free to use a Greek Delta symbol in your mind.)
So how much does that single in the second inning make it likelier that team will win? How much likelier is the win after the setup man got three crucial outs in the eightth? [sic]
Another question you may have....
What WPA is good for: WPA is good for telling you who deserves the game ball. WPA analysis is also good for storytelling. It gives you a statistical evidence that this was the turning point of the game, the most important play. WPA is a new-school stat in old-school garb — it tells you exactly what happened, and exactly how important it was.Go read it all and be informed.