During the course of a game, some situations are more tense and suspenseful than others. For instance, we know that a one-run lead in the bottom of the ninth inning is more suspenseful than a one-run lead in the top of the third inning. Batting with two runners on and two outs in the eighth inning is filled with more pressure than batting in the same situation in the second inning. Leverage Index (LI) is merely an attempt to quantify this pressure so we can determine if a player has been used primarily in high-leverage or low-leverage situations.
There are many different iterations of LI, including:
pLI: A player’s average LI for all game events.
phLI: A batter’s average LI in only pinch hit events.
gmLI: A pitcher’s average LI when he enters the game.
inLI: A pitcher’s average LI at the start of each inning.
exLI: A pitcher’s average LI when exiting the game.
An average (or neutral) LI is 1. High leverage is 1.5 and above, and low leverage is below 1. 10% of all real game situations have a LI greater than 2, while 60% have a LI less than 1.
Things to Remember:
● Leverage Index depends on the inning, score, outs, and number of runners on base.
● If you go to a player’s “Splits” section his FanGraphs player page, you can find how the player performed in low, medium, and high leverage situations. While some players may have performed well in high-leverage situations compared to their average performance, that does not necessarily mean they will continue to produce that way in the future. “Clutch hitting” is generally the result of small sample sizes and random variation. A player shown to be very clutch one season does not necessarily mean that he will be very clutch in the next.
Links for Further Reading: