From BR Bullpen
Every time a batter completes a turn batting, he is credited with a plate appearance (PA). A player is considered to have completed a turn batting when he reaches base safely via a hit, fielder's choice or an error, is awarded first base via a base on balls, hit by pitch or interference/obstruction (including catcher's interference), or is retired before reaching base. If a turn at the plate is interrupted by an inning ending on a caught stealing or other similar event that prevents the completion of the batter's turn at the plate, then no PA is charged and the batter leads off the following inning with a fresh count. Similarly, if a game ends on a balk, the player who was batting at the time of the balk is not charged with a PA. If a batter is replaced during a time at the dish, the count at the time is he replaced is used to determine whether or not he is charged with a PA. For statistical purposes, plate appearances that result in a batter reaching base via obstruction or interference are ignored when calculating on-base percentage.
A plate appearance is different from an at bat, which is a sub-category of plate appearances and excludes certain outcomes including, but not limited to, a base on balls or hit by pitch. The number of at bats is used to calculate batting average and slugging percentage, while the number of plate appearances is used to calculate a batter's on-base percentage (noting the aforementioned exclusions).
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 Further Reading
- Brian Yonushonis: "The Infinitely Long MLB Plate Appearance, SABR, Volume 40, Number 1 (Spring 2011), pp. 103-107.