Most sabermetric statistics are context neutral — they do not consider the situation of a particular event or how some plays are more crucial to a win 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 to that win 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 per-play basis.
For example, 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, pushing the Rays’ win expectancy jumps to 75%. That difference in win expectancy (in decimal form, +.30) from the beginning of the play to the end is Ben Zobrist’s WPA for that play. If Zobrist strikes out during his next at bat and lowers his team’s win expectancy by 5%, his overall WPA for the game so far would be +.30 – .05 = +.25, as WPA is a counting statistic and is additive.
Technically, WPA values for events that contribute positively to a win can range from about 2% (.02 WPA) to 95% (.95 WPA). The extreme swings in WPA are not terribly common, just as walk-off home runs are exciting events we don’t see every day.
Cumulatively, season-long WPA is not predictive, making it an ineffective number for projections of a player’s talent. However, it is a good describer of what happened in the game and how a win was achieved. And since +1 WPA equals 100% in win expectancy, +1 WPA is the equivalent of one win.
For MLB regulars, here’s a quick breakdown on season-long WPA scores:
Things to Remember:
● WPA is not highly predictive. Generally, it is not used for player analysis and projecting the future. But it does give us a picture of which players helped their team the most during the course of a game. A fun way to think of WPA is as a storytelling statistic. It highlights the big (and most exciting) moments of a game as well as the players who contributed most to a win (or loss).
● Like RBIs and HRs, WPA is a counting statistic, meaning that players with more playing time will have more opportunities to accrue a higher WPA.
Links for Further Reading: