Baseball is filled with statistics, rules, and archaic terms that can often form what sounds like a foreign language.The Sons of Sam Horn glossary provides a better understanding of these terms through straightforward definitions, clear explanations, and examples pulled straight from the baseball world. If there is anything you would like us to add to our glossary, please contact us.
Leverage Index, or LI, quantifies the potential influence one play has on the outcome of a game, given the score, inning, and base/out situation. LI is based on the changes in win expectancy for every possible outcome of a play and the resulting win expectancy from each outcome.
Created by Tom Tango, the way LI changes throughout every situation can be seen here – though it is important to note that LI does change slightly based on the run-scoring environment since it is based on WE. An average leverage index is 1.0, while anything below 0.85 is considered a low-leverage situation. A high-leverage situation is above 2.0 and defined by Tango as a “specific situation where the impact of an event will be much greater than in a typical situation.”
Since leverage index measures the impact each play can have, it is not a reflection of talent. However, it does give us a view into how much, or little a manager, and perhaps an organization, believes in a player.
Leverage index helps put a number on the important moment in a game and is an important component of the WPA charts used by both Baseball-Reference and Fangraphs as seen below:
As this Baseball-Reference chart from the series-deciding Game 6 of the 2013 World Series shows, the last hi-lev situation (light red) occurred before the bottom of the fourth inning, as the Boston Red Sox jumped to an early lead. However, in the top of the seventh inning, John Lackey walked Matt Holliday to load the bases. Junichi Tazawa was then called upon and he forced Allen Craig to ground out. The leverage index for that particular plate appearance was 1.2, and wasn’t considered high because the Red Sox were up five runs with two outs.
Here is what the same game looks like with the Fangraphs WPA chart:
Types of LI
Leverage index is further broken down into a few types:
- pLI: A player’s average leverage index.
- phLI: A player’s average leverage index when appearing as a pinch hitter.
- gmLI: A pitcher’s average leverage index when he enters the game. This can show how much a manager believes in a reliever. A reliever with a high gmLI is often coming into bad situations and is trusted to put out fires.
- inLI: The average leverage index when the pitcher starts an inning.
- exLI: The average leverage index when he exits the game. The difference between these last two can show if a pitcher is adept at either starting or putting out fires.