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.
Hitters with high on-base percentages generally bat toward the top of the lineup, as their ability to get on base makes them prime candidates to be driven in by power hitters in the middle of the order. Teams also prefer to bat high-OBP hitters near the top of the order to ensure that the players who get on base most often receive more plate appearances.
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.
Some fantasy scoring systems count on-base percentage in lieu of batting average. But regardless of a league's offensive-rate stat of choice, OBP tends to correlate with runs scored. And because Major League front offices value OBP highly, low-average hitters often receive their ample share of playing time -- and, thus, opportunities to accumulate fantasy counting stats -- as long as they walk enough to post satisfactory OBPs.
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