In softball and baseball statistics, on-base percentage (OBP); sometimes referred to as on-base average/OBA, as the statistic is rarely presented as a true percentage) is a measure of how often a batter reaches base for any reason other than a fielding error, fielder's choice, dropped/uncaught third strike, fielder's obstruction, or catcher's interference (the latter two are ignored as times-on-base (TOB). OBP is added to slugging average to determine on-base plus slugging (OPS). It first became an official MLB statistic in 1984.
The on-base percentage of all batters faced by one pitcher or team is referred to as on-base against.
Traditionally, players with the best on-base percentages bat as leadoff hitter, unless they are power hitters, who traditionally bat slightly lower in the batting order. The league average for on-base percentage in Major League Baseball has varied considerably over time; at its peak in the late 1990s, it was around .340, whereas it was typically .300 during the dead-ball era. On-base percentage can also vary quite considerably from player to player. The record for the highest career OBP by a hitter, based on over 3000 plate appearances, is .482 by Ted Williams. The lowest is by Bill Bergen, who had an OBP of .194.
On-base percentage is calculated using this formula:
NOTE: Sacrifice flies were not counted as an official statistic until 1954. Before that time, all sacrifices were counted as sacrifice hits (SH), which included both sacrifice flies and bunts. Sacrifice bunts (sacrifice hits since 1954), which would lower a batter's on-base percentage, are not included in the calculation for on-base percentage, as bunting is an offensive strategy – often dictated by the manager – the use of which does not necessarily reflect on the batter's ability and should not be used to penalize him. For calculations of OBP before 1954, or where sacrifice flies are not explicitly listed, the number of sacrifice flies should be assumed to be zero.
It is possible for a player's on-base percentage to be lower than his batting average (H/AB). This is because sacrifice flies are counted for the purposes of OBP, but not batting average. For example, a player who has 2 hits in 6 at-bats plus a sacrifice fly has batting average of .333, but an OBP .286. The player who experienced this phenomenon with the most number of at-bats over a full season was Ernie Bowman. In 1963, with over 125 at-bats, Bowman had a batting average of .184 and an on-base percentage of .181.
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