Win Probability Added (WPA): this is a statistic that takes every play during every game into the context in which it occurred. If you hit a grand slam home run in the 9th inning of a game in which your team is leading 10-0, you’ve added virtually nothing to the likelihood that your team will win that game. If, on the other hand, you do it with two outs in the bottom of the 9th inning and your team trailing by three runs, you’ve turned a almost certain defeat into a victory.

On this site, we will most often use WPA with respect to clutch post-season performances and also, for the regular seasons, to relief pitchers. Because they pitch so much fewer innings than starters, the value of relievers can’t be measured by Wins Above Replacement (WAR), which is a “counting” stat in which total innings matters. WPA is especially important for closers because it shows the impact of the player’s performance on his team’s chance to win the game.

WPA is a “counting” stat that can count backwards too. If you’re a closer and you enter a game with a 3-run lead in the 9th, you should close out that game over 96% of the time so, depending on the ballpark and the season, you would get approximately 0.04 WPA “points” for bringing your team from a 96% win probability to 100% (an actual win). If, on the other hand, you give up four runs and lose that game, you’ll get -0.96 WPA “points” if it’s a road game. (If you’re pitching at home, your team still has a chance to come back to win so you can’t go from 96% to 0%).

Now, if you enter the game in the 9th with just a one-run lead and convert the save, that’s worth anywhere between 0.16 and 0.24 WPA. Why such a big difference? It’s because of the ballpark and the season. Using Trevor Hoffman as an example, a 9th inning save with a one-run lead at PetCo Park was worth about 0.16 WPA. A 9th inning save with a one-run lead at Coors Field was worth about 0.24 WPA because it’s much easier to score runs at Coors Field.

To help you understand how it works (and impacts relief pitching), here are some famous examples from October baseball with the relief pitcher involved and the positive or negative impact they had on each game.

Courtesy Baseball Reference
PitcherTeamYearRoundGameWhen they entered, what happenedWPA
Madison BumgarnerSF2014WS7Bot 5th, 3-2 lead, throws 5 scoreless IP for save & Series win0.60
Sparky LyleNYY1977LCS4Bot 4th, 2 runners on, 5-4 lead, 5.1 scoreless IP, gets the win0.57
Rollie FingersOAK1973WS7Bot 8th, 3-1 lead, 2 RISP, 0 out, sac fly & 6 outs to clinch Series0.46
Donnie MooreCAL1986LCS5One out from WS, 2-run HR to Dave Henderson, Sac Fly in 11th-0.91
Dennis EckersleyOAK1988WS1Bot 9th, 4-3 lead, 2-out, 2-run HR to Kirk Gibson-0.83
Mitch WilliamsPHL1993WS6Bot 9th, 6-5 lead, 3-run HR to Joe Carter to end the Series-0.79