Win probability added

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Win probability added (WPA) is a sport statistic which attempts to measure a player's contribution to a win by figuring the factor by which each specific play made by that player has altered the outcome of a game. It is used for baseball and American football. Some form of win probability has been around for about 40 years; however, until computer use became widespread, win probability added was often difficult to derive, or imprecise. With the aid of Retrosheet, however, win probability added has become substantially easier to calculate. The win probability for a specific situation in baseball (including the inning, number of outs, men on base, and score) is obtained by first finding all the teams that have encountered this situation. Then the winning percentage of these teams in these situations is found. This probability figure is then adjusted for home-field advantage. Thus win probability added is the difference between the win probability when the player came to bat and the win probability when the play ended. Some people confuse win probability added with win shares, since both are baseball statistics that attempt to measure a player's win contribution. However, they are quite different.