Library Update: Leverage Index

It’s not uncommon to hear the word “leverage” in a baseball conversation these days. While it used to be reserved for the analytical corners of the community, the term has gained mainstream popularity as a representation of how important a particular situation is during a game. We debate the use of relievers in high leverage situations and talk about how certain hitters have performed well in certain contexts. Leverage is all the rage.

At FanGraphs, we host a metric that quantifies the concept of leverage, called Leverage Index (LI). Simply put, it measures the possible change in win expectancy for that plate appearance relative to the average change in win expectancy for all plate appearances. When the bases are loaded in a 2-1 game in the 9th inning, the LI is very high. If they’re loaded in a 14-2 game in the 6th inning, the LI is very low. You can read a more technical breakdown of the metric and its uses at the newly updated Library page concerning Leverage Index.

As always, feel free to inquire about the metric in the comments section, to find me on Twitter @NeilWeinberg44, or to stop by our weekly FanGraphs Q&A, Wednesdays at 3pm, if you want to learn more.

Neil Weinberg is the Site Educator at FanGraphs and can be found writing enthusiastically about the Detroit Tigers at New English D. Follow and interact with him on Twitter @NeilWeinberg44.

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There’s about 10% of PA that are noted as high leverage on Fangraphs. This would imply an LI of at least 2.0, not 1.5. Baseball Reference uses 1.5, and it has about 20% of PA as high leverage.