OBP refers to how frequently a batter reaches base per plate appearance. Times on base include hits, walks and hit-by-pitches, but do not include errors, times reached on a fielder's choice or a dropped third strike. (Separately, sacrifice bunts are removed from the equation entirely, because it is rarely a hitter's decision to sacrifice himself, but rather a manager's choice as part of an in-game strategy.)
A hitter's goal is to avoid making an out, and on-base percentage shows which hitters have accomplished that task the best.
On-base percentage can also be applied as an evaluative tool for pitchers, although this is done less frequently. In such cases, it is referred to as on-base against.
On-base percentage was a statistic invented in the 1940s-50s by Dodgers executive Branch Rickey and statistician Allan Roth. It did not become an official statistic until 1984.
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