On base percentage
From BR Bullpen
On Base Percentage (aka OBP, On Base Average, OBA) is a measure of how often a batter reaches base. It is approximately equal to Times on Base/Plate appearances.
The full formula is OBP = (Hits + Walks + Hit by Pitch) / (At Bats + Walks + Hit by Pitch + Sacrifice Flies). Batters are not credited with reaching base on an error or fielder's choice, and they are not charged with an opportunity if they make a sacrifice bunt.
|All Time Leaders|
There is also an Adjusted statistic called OBP+, which adjusts the OBP to account for the ballpark and the league that the player played in. In addition, the number is "normalized", so that the median is 100, with better-than-average scores above 100. The formula is OBP+ = 100*(OBP/lgOBP), with lgOBP representing the league average for that year.