Onbase percentage
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In baseball statistics, onbase percentage (OBP; sometimes referred to as onbase average/OBA, as the statistic is rarely presented as a true percentage) is a statistic generally measuring how frequently a batter reaches base.^{[1]} Specifically, it records the ratio of the batter's timesonbase (TOB) (the sum of hits, walks, and times hit by pitch) to their number of plate appearances.^{[1]} It first became an official MLB statistic in 1984.
By factoring in only hits, walks and times hit by pitch, OBP does not credit the batter for reaching base due to fielding errors or decisions, as it does not increase when the batter reaches base due to fielding error, fielder's choice, dropped/uncaught third strike, fielder's obstruction, or catcher's interference.
Onbase percentage is added to slugging average to determine onbase plus slugging (OPS). The onbase percentage of all batters faced by one pitcher or team is referred to as onbase against.
Overview
Traditionally, players with the best onbase percentages bat as leadoff hitter, unless they are power hitters, who traditionally bat slightly lower in the batting order. The league average for onbase 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 deadball era. Onbase 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.
Onbase percentage is calculated using this formula^{[2]}^{[3]}^{[4]}:
where
 H = Hits
 BB = Bases on Balls (Walks)
 HBP = Hit By Pitch
 AB = At bat
 SF = Sacrifice fly
In certain unofficial calculations, the denominator is simplified and replaced by Plate Appearance (PA); however, the calculation PAs includes certain infrequent events that will slightly lower the calculated OBP (i.e. catcher's interference, and sacrifice bunts).^{[4]} Sacrifice bunts are excluded from consideration on the basis that they are usually imposed by the manager with the expectation that the batter will not reach base, and thus do not accurately reflect the batter's ability to reach base when attempting to do so.^{[5]}
Alltime leaders
bold is active player
Singleseason leaders
#  Player  OBP^{[7]}  Team  Year 
1  Barry Bonds  .6094  San Francisco Giants  2004 
2  Barry Bonds  .5817  San Francisco Giants  2002 
3  Ted Williams  .5528  Boston Red Sox  1941 
4  John McGraw  .5475  Baltimore Orioles  1899 
5  Babe Ruth  .5445  New York Yankees  1923 
6  Babe Ruth  .5319  New York Yankees  1920 
7  Barry Bonds  .5291  San Francisco Giants  2003 
8  Ted Williams  .5256  Boston Red Sox  1957 
9  Billy Hamilton  .5209  Philadelphia Phillies  1894 
10  Babe Ruth  .5156  New York Yankees  1926 
See also
 List of MLB players with a .400 onbase percentage
 Moneyball: The Art of Winning an Unfair Game
 The OnBase Specialist: A New Model for Baseball Offense
Notes
 ^ ^{a} ^{b} MLB Glossary
 ^ [http://www.baseballreference.com/leaders/onbase_perc_career.shtml Baseball reference: OBP
 ^ Beyond the Box Score
 ^ ^{a} ^{b} Fangraphs

^ Cite error: The named reference
glossary
was invoked but never defined (see the help page).  ^ "Career Leaders for On Base Percentage". Sports Reference, Inc. Retrieved 20110625.
 ^ "Single Season League Leaders for OnBase Percentage". Sports Reference, Inc. Retrieved 20110625.