Taken directly from the FanGraphs Glossary, 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. While venturing to a website like this implies prior knowledge of WPA I always like to err on the unknowing side. Since baseball is zero-sum game, everything positive is cancelled out by something negative; a home run is very positive for the hitter but equally negative for the pitcher. Due to this, the WPA of a winning team in any given game will add up to +.500; that of a losing team will be -.500.
Keeping this in mind, let’s take a look at this afternoon’s Diamondbacks/Giants game:
The matchup of Brandon Webb and Barry Zito ultimately resulted in a 4-1 Diamondbacks victory and, according to WPA, it was pretty much all due to Brandon Webb. The former Cy Young Award winner went for 8 IP, 3 H, 1 ER, 2 BB, 5 K, and a pitching WPA of +.337. On top of that, as evidenced by the graph above, Webb also added a two-run single. His hit put the DBacks ahead 2-0 and turned out to be the biggest play of the game; the WPA for the single play was +.165. Since the hit proved to be so significant Webb ended with a batting WPA of +.210.
Add both of them up and Webb’s WPA comes to +.547. As mentioned at the start, the net sum WPA of a winning team will be +.500.
Webb not only shut the Giants down for eight innings of great pitching but added the biggest offensive play of the game; the LI of his at bat was 2.96. Of course, watching the game or highlights of it would explain that it was not a laser sharp single, but the fact is that the blooper fell in, two runs scored, and that was all Webb needed.
The problem with looking solely at WPA in this case is that Webb’s single came with the bases loaded so he could not have knocked in the runs without his teammates doing their part to get on base. This is where WPA/LI comes in. The stat also goes by the nickname “context neutral wins” and tracks the contributions with the Leverage Index aspect removed. It is not calculated by dividing the overall WPA by the overall LI but rather dividing the WPA and LI of every single play and then adding everything together. With regards to this particular game, and Webb’s single in particular, the WPA and LI were high because the situation existed as a result of others getting on base. Webb had nothing to do with them reaching base and so using WPA to determine whether or not he single-handedly beat the Giants would not necessarily be accurate; the WPA/LI would determine exactly what he individually contributed to the game.
Looking at the WPA and LI of each play in this game, Webb’s WPA/LI from a pitching standpoint adds up to +.328, not much different than the +.337 WPA. His hitting, however, lessens quite a bit due to the context that padded his batting WPA. From a context neutral standpoint, his batting WPA/LI comes to +.067. Though still qualifying as a positive contribution it is much less than the +.210 WPA. Overall, adding them together, Webb’s WPA/LI for this game was +.395. While he did not single-handedly beat the Giants he would definitely win the Game MVP award for contributing much more to his team’s victory than anyone else.