Introducing Statistics: On-Base Plus Slugging
Hitting can be broken down into two basic components: the ability to get on base and the ability to hit for power. These have been measured in different ways for years. The oldest measure of hitting is the batting average, which is the rate at which a batter gets a hit (BA=Hits/At Bats). It ignores times at plate that result in walks or hit batsmen. Though either outcome involves a batter getting on base, it is not done through a hit and thus is ignored. In 1876, the first year of the National League, walks were recorded as outs and thus depressed batting average. Batting average, it appears, has three major flaws: It measures the rate of an occurence, meaning that a batting average of .333 is the same if we are talking about 1 hit in 3 at bats or 33 in 99. It ignores other opportunities to get on-base. It also has no measure of how good a hit occurs.
On-Base Percentage is an attempt to correct for the second of these errors. OBP=(Hits+Walks+Hit Batsmen)/(At Bats+Walks+Hit Batsmen+Sacrifices+Sacrifice Flies). This formula brings all plate appearances into the equation (in the denominator) instead of an arbitrarily limited number of at bats. It also treats all times that a batter gets on base as valuable. (It does ignore if a batter reaches on an error or on a fielder’s choice, probaby because those are not considered products of the hitter’s skill.) However, it waits all times on base equally, which seems intuitely mistaken. A double is better than a walk for two reasons: It gets a batter one base further, and it has a better chance of advancing preceding baserunners. Nevertheless, OBP does correct one flaw of batting average.
Slugging percentage attempts to correct batting average’s third flaw. SLG=(Total Bases/At Bats). The denominator is the same as battign average, but the numerator is total bases, i.e. how many bases a batter gets from all of his hits combined. Slugging percentage ignores other ways to get on base, focusing solely on the power of hits. It has the advantage of treating a home run as more valuable than a single, but it still ignores the importance of getting on base without a hit. To solve the flaws of OBP and SLG, they are added together to create OPS.
OPS=OBP+SLG. This stat is probably the most visible sabermetric batting statistic. It creates a scale that gives weight to all times on base and to the value of hits beyond singles. However, it has the weakness of all rate stats. It measures how often a batter does its given elements, and not how many. While it might be useful to know that a batter reaches base 40% of the time, it also helps to know how many times he reaches base. An OPS of 1.000 is outstanding, but and OPS of .990 in twice as many plate appearances is more valuable to a team. Once again, health is important in measuring skill. It is not sufficient to make an OPS of .500 as valuable as an OPS of 1.000 if the lesser player is twice as healthy. But it is incredibly important in deciding tough cases.