Get to Know: WPA

WPA (win probability added): WPA is the difference in win expectancy (WE) between the start of the play and the end of the play. That difference is then credited/debited to the batter and the pitcher. Over the course of the season, each players’ WPA for individual plays is added up to get his season total WPA.

Calculation Example: In game 4 of the 2007 World Series, the WE for the Rockies started out at 50%. When Jacoby Ellsbury doubled off Aaron Cook in the very first at-bat in the game, the Rockies WE declined to 44.2%. The difference or WPA was .058 wins (5.8%). Ellsbury was credited +.058 wins and Aaron Cook credited with -.058 wins.

Why you should care: WPA takes into account the importance of each situation in the game. A walk off home run is going to be weighted more then a home run in a game that has already gotten out of hand. This makes it a great tool for determining how valuable a player was to his team’s win total.

When not to use it: WPA is more of a descriptive statistic and not that great of a predictive statistic. There are better statistics to use in raw player evaluations than WPA.

Links and Resources:

The Hardball Times: The One About Win Probability
The Book Wiki: Win Probability Added
Wikipedia: Win Probability Added
WPA is… WPA is not…




Print This Post



David Appelman is the creator of FanGraphs.


One Response to “Get to Know: WPA”

You can follow any responses to this entry through the RSS 2.0 feed.
  1. Andy says:

    In the “Why you should care” section, it should be “more than” rather than “more then.”

    +7 Vote -1 Vote +1

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>