On-base percentage
In baseball statistics, on-base percentage (OBP) measures how frequently a batter reaches base. An official Major League Baseball (MLB) statistic since 1984, it is sometimes referred to as on-base average (OBA), as it is rarely presented as a true percentage. Generally defined as "how frequently a batter reaches base per plate appearance", OBP is specifically calculated as the ratio of a batter's times on base (the sum of hits, bases on balls, and times hit by pitch) to the sum of at bats, bases on balls, hit by pitch, and sacrifice flies. OBP does not credit the batter for reaching base on fielding errors, fielder's choice, uncaught third strikes, fielder's obstruction, or catcher's interference. OBP is added to slugging average (SLG) to determine on-base plus slugging (OPS). The OBP of all batters faced by one pitcher or team is referred to as "on-base against". On-base percentage is calculable for professional teams dating back to the first year of National Association of Professional Base Ball Players competition in 1871, because the component values of its formula have been recorded in box scores ever since. (Wikipedia).
