WE (win expectancy): The percent chance a particular team will win based on the score, inning, outs, runners on base, and the run environment.
Assumptions: Win expectancy as it’s currently calculated assumes that each team has an equal chance of winning at the start of a game.
Specifics: FanGraphs uses Tangotiger’s most current win expectancy tables which are available for 3.0 to 6.5 run environments in increments of .5 runs. The league average run environment is used to calculate win expectancy. When the run environment falls in between a .5 increment, the tables are then weighted accordingly to achieve the correct win expectancy.
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