Win Probability Added
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Stat of the Week
by Peter Liubicich
Win Probability Added
This post was written by Peter Liubicich on January 7, 2013
Posted Under: Stat of the Week

In continuing our understanding of Clutch, this week we will look at Win Probability Added.  Last week we were introduced to Leverage Index, and to remind you of the process we’re going through here is FanGraphs’ formula to determine how “clutch” a player is throughout a season.

Clutch = (WPApLI) - WPA/LI

To understand Win Probability Added (WPA) we first need to take a quick look at Win Expectancy (WE).  WE is “the percent chance a particular team will win based on the score, inning, outs, runners on base, and the run environment.”  Historical data is used to see the percent chance a team has of winning based on the:

  • Inning
  • Score differential
  • Outs
  • Runners on base

WE measures the team’s chances of winning a game based on the historical data of the previous factors mentioned and WPA tracks how an individual player affects the WE of his team on a per-play basis.

FanGraphs provides an example to put WPA into practice:

“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.”

Some notes on a player’s WPA throughout a season are that it is not predictive, but it does help indicate how a player helped their team win (or lose) a game.  In evaluating a player’s season long WPA here is a chart, with 1 WPA being equivalent to one win.

Rating

WPA

Excellent

+6.0

Great

+3.0

Above Average

+2.0

Average

+1.0

Below Average

0.0

Poor

-1.0

Awful

-3.0

On Thursday we’ll look at the player’s with the highest WPA from this past season as we try to understand clutch even better.

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  1. WPA/LI  on January 14th, 2013 @ 5:47 pm

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