Definition of On-Base Percentage in Baseball
Definition 1 2
The ratio that indicates how often a batter reaches base at the plate is called On-base Percentage, or OBP for short. In terms of Hits H, Walks BB and Hit By Pitch HBP, At Bats AB, and Sacrifice Flies SF, it is defined as follows. $$ \begin{align*} OBP :=& {{ H + (BB + HBP) } \over { AB + (BB + HBP) + SF }} \\ 출루율 :=& {{ 안타 + 사사구 } \over { 타수 + 사사구 + 희생플라이 }} \end{align*} $$
Explanation
Put very simply, if a batter has an on-base percentage of 0.400, it means that in about four out of every ten plate appearances, the batter reaches base by whatever means. Whether by getting a hit or drawing a walk, if the batter ends up on base without using up an out, it counts the same. In fact, drawing a walk means the pitcher threw at least four pitches, so it can also be viewed as an indicator of the ability to wear down the opposing pitcher’s stamina.
Why Sacrifice Bunts Are Not Included in the Denominator
Although both are sacrifice hits, a sacrifice fly goes into the denominator while a sacrifice bunt does not. The difference between a sacrifice fly and a sacrifice bunt is a difference in the batter’s mindset. Of course,
- one might, not minding getting out, take a swing that lifts the ball in order to at least hit a sacrifice fly, and
- one might lay down a bunt resigned to getting out, yet with a bit of luck turn it into a bunt hit,
but from a strategic point of view, the two cannot be placed on exactly the same footing. Both may equally fail to reach base and equally produce a run, but if we must split hairs, a sacrifice fly is a case of failing to reach base, whereas a sacrifice bunt is a case of choosing not to reach base.
A manager can instruct a player to “hit at least a sacrifice fly,” but does not say “hit a sacrifice fly.” If one is going to swing, one should aim for a hit; no manager demands a sacrifice fly from the outset. Conversely, a manager can instruct a player to “lay down a (sacrifice) bunt,” but cannot issue an instruction to “lay down a bunt and still survive.” That is encouragement, not a tactic.
If both of these entered the denominator of on-base percentage, then the instruction to “lay down a bunt” would become the unjust directive, “lower your on-base percentage and contribute to the team.” A sacrifice bunt is about sacrificing an out for the team; it must not be about sacrificing the player’s individual record. Of course, if you don’t know baseball well, it’s not a big problem if you fail to understand this distinction. Let’s read the following explanation.
Sabermetrics
What is absurd about on-base percentage is that, for a metric whose very purpose is to calculate how often a batter reaches base at the plate, its computation formula is strangely complicated. If we go by the conceptual definition that preceded the formula—that is, “a ratio meant to represent how often a batter reaches base at the plate”—then the following definition really should have been sufficient. $$ 출루율 \overset{?}{=} {{ 안타 + 사사구 } \over { 타석 }} $$ However, letting $X$ denote reaching base due to the defense’s batting interference/base-running interference, Plate Appearances PA satisfy the following equation. $$ \begin{align*} 타석 =& 타수 + 사사구 + 희생타 + X \\ =& 타수 + 사사구 + (희생플라이 + 희생번트) + X \end{align*} $$ For the reasons mentioned above, however, sacrifice bunts drop out, and $X$ occurs so rarely as a record that it is fine to treat it as $X \approx 0$. Even if it were to occur frequently, there is ample justification for $X$ not being included in the computation of on-base percentage. In a batter’s records, a higher number of plate appearances is an indicator by which we can judge durability or diligence, and since it is unreasonable for an opponent’s mistake to reduce these plate appearances, it should merely be added to the plate-appearance record. When computing on-base percentage, it can still be regarded as a meaningless number and reasonably removed, and this corrected on-base percentage works out, just as introduced in the definition, as follows. $$ 출루율 = {{ 안타 + 사사구 } \over { 타수 + 사사구 + 희생플라이 }} $$ If, having seen this derivation, you can accept the complicated computation formula, then reading this post has been worthwhile. Since batters usually do not lay down an enormous number of sacrifice bunts, unless there are meaningfully many sacrifice bunts, on-base percentage ultimately reduces to its original conceptual approximation, the ratio of reaching base per plate appearance, as follows. $$ 출루율 \approx {{ 안타 + 사사구 } \over { 타석 }} $$
Moneyball
Along with sabermetrics, the credit for making on-base percentage famous to the public as a baseball statistic largely goes to “Moneyball.” Moneyball is a film that dramatizes a real case in which a strong team was built by applying sabermetrics; roughly summarized, its story is about trying to solve the problem of reinforcing a financially struggling club’s roster by gathering players who had high on-base percentages but were undervalued.
In the film, too, the characters have many lines emphasizing on-base percentage, such as “So what matters in the end? On-base percentage.” But one thing that must not be misunderstood now is that the film’s setting is a point in time when sabermetrics had not yet become as widespread as it is today, and on-base percentage was not yet highly valued. The situation the protagonist’s team faces in the film is as follows:
- They have no money.
- The team’s core players are all leaving.
- On-base percentage is the most undervalued statistic in the league.
- There are many players with high on-base percentages but low salaries.
In other words, the film’s content is about being ahead of its time in accurately evaluating players and building the strongest team at minimal cost in order to win—not a success myth about buying up players with good on-base percentages and winning it all. To put it bluntly, you cannot build a strong team on on-base percentage alone. After that, every club came to know the importance of sabermetrics, and today most statistics receive a relatively fair evaluation. In other words, on-base percentage today has had its price raised to a fair level, so a high on-base percentage is not unconditionally a good thing.