The baseball statistic WPA stands for Win Probability Added. This is an interesting statistic in that it identifies how much an individual player is helping or hurting his team’s pursuit of winning games.
We all know that hitting a home run is going to help the team, but how much? And do all home runs help the team to the same degree?
Thanks to WPA, we can see that the actual value added to the team depends greatly on the context of each event.
The Concept of WPA
There are plenty of statistics that capture a portion of what a player has added to his team’s chances to win. Whether they are counting stats like home runs and strikeouts, or rate stats like batting average and ERA, we don’t lack for ways to evaluate pieces of a players performance.
What most of those other statistics lack, however, is context.
A bases-empty single in the ninth inning of a blowout is treated the same as a clutch go-ahead single in a tie game from a batting average perspective. By using WPA, we can provide context to a player’s performance over the course of a year, quantifying which players are doing the most to lead their team to victory.
Why is WPA Important?
Statistics can be cold, and some people are turned off from advanced metrics because they ‘take the fun out of the game’. While there is a lot to be learned from modern baseball statistics, it is true that they look at the game in a matter-of-fact manner, skipping some of the romantic aspects of the game from years gone by.
In an interesting way, the advanced statistic of WPA can bridge that gap.
This is a stat that really isn’t designed for predictive value, but rather to help tell a story of which players were most important in the biggest moments.
For instance, we have included a chart below that highlights the top ten batters for the 2018 season in terms of WPA. Coming in at 7th on the list is Andrew Benintendi of the Boston Red Sox with a WPA of 3.84. However, when you look at another offensive statistic, wRC+, Benintendi only ranks 48th for the 2018 season with a wRC+ of 122.
That’s still a very good mark, but it doesn’t place him up in the top ten across MLB. So, in this way, WPA is able to ‘tell the story’ that Benintendi came up with big hits in important situations frequently during the 2018 season.
How is WPA Calculated?
WPA is based on changes in win expectancy. The difference in win expectancy from the start of a plate appearance to the end is going to account for the WPA for that plate appearance (for both pitcher and batter). Each at-bat will have a total WPA of 0, as the positive or negative value produced by the batter will always be canceled out by the inverse value assigned to the pitcher.
It will be easier to understand how this works with the help of an example:
On June 24th, 2019, the Toronto Blue Jays visited New York for a game against the Yankees. Through 4.5 innings, the Blue Jays led 2-0. In the bottom of the fifth, the Yankees put together back-to-back one-out singles to bring Aaron Hicks to the plate with runners on first and second. At the beginning of this at-bat, the Yankees had a 34.8% chance to win the game, since they trailed by two runs. However, Hicks hit a three-run homer off Toronto starter Aaron Sanchez, and flipped the game around. At the end of the at-bat, the Yankees had a 68.5% chance to win the game.
Some basic math using the information above can provide the WPA data for both Hicks and Sanchez in that at-bat. Hicks adds .337 to his total, which is the difference between the pre- and post-win expectancy numbers, while Sanchez loses .337 from his total. Most at-bats will not feature such large WPA values, but this was a pivotal home run that completely shifted the likely outcome of the game.
What is a Good WPA?
Over the course of a full season, a player with a WPA total of 2.0 or more has contributed nicely to his team’s efforts. Those who get up into the 4.0 – 6.0 range had tremendous seasons and were likely responsible for some highlight moments in key spots.
To give you an idea of what the WPA rankings look like in a given season, here are the top ten batters and pitchers from 2018:
WPA Batting Leaders, 2018 Season
WPA Pitching Leaders, 2018 Season
What are the Problems with WPA?
Much as is the case with Leverage Index, there can be a temptation to use WPA as a predictive statistic, and that is a mistake.
While it is interesting to review WPA to see which players have provided the most help to their team over a given time period, that information should not necessarily be seen as any kind of prediction for the future. There is no statistical indication that some players are ‘more clutch’ than others. Rather, the better players will be more likely to perform well in the clutch, because they are more likely to perform well in any situation.