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Leverage measures how important the situations a reliever has been used in are. A leverage of 1.00 is the same importance as the start of a game. Leverage values below one represent situations that are less important than the start of a game (such as mopup innings in a blowout). Leverage values above one represent situations with more importance (such as a closer protecting a one-run lead with bases loaded in
the 9th inning).
Mathematically, leverage is based on the win expectancy work done by Keith Woolner in BP 2005 (read an explanation by Dan Fox here), and is defined as the change in the probability of winning the game from scoring (or allowing) one additional run in the current game situation divided by the change in probability from scoring
(or allowing) one run at the start of the game.