WPA:MVP as LI:GOD

A few people in the word (read: one) may have noticed that I did not post either an MVP or Game O' the Day for Monday, April 15.  While those posts were driving great traffic, my intention was to bring awareness to two statistics that tell the story of an MLB game, not necessarily recap each night of baseball.

WPA:MVP

Win Probability Added (WPA) is a stat that measures a players impact on the game by measuring how their offensive performance impacts their team's win expectancy.  Here is an example from Fangraphs: 

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.


Basically, WPA tells you whose batting outcomes had the biggest impact on the game.  Therefore, the timing and context of the plate appearances are very important.  Thus confirming what we already know but is hard to capture in the box score: a 2-run game winning homerun has much more impact than a grand slam when you are up by 10.

For me, WPA is the best way to evaluate a player's performance on a game-by-game basis and it is also interesting to look at over a whole season (The 2012 season leader in WPA was Mike Trout with 5.32 WPA).

LI:GOD

Leverage Index (LI) captures the change in win probability resulting from the game action.  Here is an example from Hardball Times:

If there is a game with one team leading by ten runs, the possible changes in win probability, whether the event is a home run or a double play, will be very close to negligible. That is, there won't be much swing in any direction.  But, in a late and close game, the change in win probability among the various events will have rather wild swings. With a runner on first, two outs, down by one, and in the bottom of the ninth, the game can hinge on one swing of the bat—a home run and an out will both end the game, but with vastly different outcomes for the teams involved. 

Blowouts are going to have a low average LI (aLI) because there is not much change in the probability in who will win the game.  Therefore, games with a high average LI are going to be the most interesting game of the night because they will have gone back and forth without one team really taking control of the game.

Fangraphs posts live WPA and LI information for every game.  So, instead of having me summarize every night in the MLB take a look for yourself.