Win Probability Added

Win Probability Added is a technical baseball statistic which attempts to measure a player's win contribution by figuring how much each specific play he made altered the outcome of a game.

Some form of Win Probability has been around for about 40 years; however, until computer use became widespread, Win Probability Added was often hard to figure 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 (i.e., men on second and third, two out) 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. Win Probability Added, thus, is the difference between the Win Probability when the player came to bat and the Win Probability when the play ended.

Christopher Shea has invented a Win Expectancy Finder to look up the actual Win Probability of every base/out, inning and score combination of all Major League games from 1979 to 1990. Chris used Retrosheet data that had been compiled by Phil Birnbaum, and his WE Finder simply looks up the percent of times a team in a given situation went on to win the game during those years.

[edit] Win Probability and Win Shares

Some people confused Win Probability Added with Win Shares, since both are technical baseball statistics that attempt to measure a player's win contribution. However, they are quite different. In Win Shares, a player with 0 Win Shares has contributed nothing to his team; in Win Probability Added, a player with 0 WPA points is average. Also, Win Shares would give the same amount of credit to a player if he hit a lead-off solo home run that turned out to be decisive as if he hit a walk-off solo home run; WPA, however, would give vastly more credit to the player who hit the walk-off homer.

[edit] Sources

• Wikipedia
• The Hardball Times