Leverage Index
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Stat of the Week
by Peter Liubicich
Leverage Index
This post was written by Peter Liubicich on December 31, 2012
Posted Under: Stat of the Week

This week we’ll start the first installment for the statistics needed to understand the Clutch which was developed by FanGraphs.  Clutch is derived from using Wins Probability Added (WPA), Leverage Index (LI) and a player’s average Leverage Index for all game events (pLI).  All of these statistics look at the circumstances of the game, and give different weights to different situations as a solo home run in the first inning does not have the same “clutch” factor as a solo home run to win the game in the bottom of the ninth.

Leverage Index is a concept introduced to us by Tom Tango, a name I believe that should be as mainstream as Bill James’.  Tango explains LI using the example of Game 6 of the NLCS between the Marlins and Cubs which is famous for the Bartman Catch.  According to Tango, LI is the

swing in the possible change in win probability. 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.

You can spot a high-leverage situation, I can spot them, and pretty much everyone can spot many high-leverage situations. All that’s left for us to do is to quantify every single game state into a number. That number is the Leverage Index. ”

Leverage Index according to FanGraphs, is the attempt to quantify the pressure of a situation and depends on the inning, score, outs and number of runners on base.  An average LI is 1, a high leverage situation is 1.5 and a low leverage situation is below 1. Most situations in a baseball game are low leverage situations, 60% are below 1, while 10% of situations have a LI over 2.

One thing I have not found is the LI for playoff games or how the first inning of Game 7 of the World Series LI compares to the LI of a day game in July.  Check back Thursday where we’ll go through some examples from this year’s playoffs.

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  1. WPA/LI  on January 14th, 2013 @ 5:47 pm
  2. Clutch  on January 28th, 2013 @ 4:04 pm

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