We move on from batting average to discuss a much simpler statistic, as well as a weighted version of batting average. These two new statistics, along with batting average, have over the last decade come to comprise the base-level portfolio of a batter’s hitting abilities.
On Base Percentage
Conceptually, this is the simplest rate statistic for a hitter. It is the number of times a runner gets on base per plate appearance, although we do still remove sacrifice hits since that is typically a managerial decision. As an equation:
$$ \text{OBP} = \dfrac{\text{H} + \text{BB} + \text{HBP}}{\text{PA}-\text{SH}} $$
We’re adding every way a hitter can reach base without defensive errors — hits, walks (BB, which is short for “Base on Balls”), and hit-by-pitches — and dividing by their plate appearances, minus any sacrifice hits.
OBP came into the cultural consciousness with Moneyball back in the early 2000s, although a great Foolish Baseball video discusses how Earl Weaver of the Baltimore Orioles quietly did something quite similar several decades earlier. Regardless, the idea is simple: rather than valuing just hits very highly (hits are cool!), we should instead value not getting out. If you’re not amazing at hitting, but you’re still able to get a lot of walks through excellent plate discipline, you should be rewarded to some extent.
Perhaps the weirdest example of this was Juan Soto from this year, who only had a .242 batting average, but his OBP was .401. Despite only getting a hit less than a quarter of his at-bats, he still got on base just over 40% of the time. That’s an important element of his game!
In general, I’d say .400 is the baseline for a very good OBP.
Slugging Percentage
You may know that a slugger is someone who hits the ball really hard, often for extra bases or home runs. As such, the slugging percentage (SLG) is a weighted batting average where instead of measuring total hits per at-bat, we measure total bases per at-bat.1
To break it down among singles, doubles, triples, and home runs (1B, 2B, 3B, HR respectively), we get this equation:
$$ \text{SLG} = \dfrac{1\cdot\text{1B} + 2\cdot\text{2B} + 3\cdot\text{3B} + 4\cdot\text{HR}}{\text{AB}}$$
However, it’s simpler to think of it as “total bases per at-bat”. So, someone with a .500 SLG gets, on average, half a base per at-bat, which would be quite good.
Here are a few fun statistics for OBP and SLG. We’ll only be considering the Live Ball Era, which began in 1920.2
The highest season OBP by a qualified player with a BA less than .300 was Mark McGwire in 1998, who had a .299 BA and a .470 OBP. (When you hit 70 home runs, a new MLB record at the time, you’ll get walked a lot.) 3
The lowest season OBP by a qualified player with a BA greater than .300 was Shawon Dunston in 1997. He had a .300 BA on the dot, yet only eked out a .312 OBP. He struck out 75 times, and only walked 8 times. That’s pretty abysmal.
The highest season SLG by a team that won the World Series was the 1927 Yankees, with a .488 SLG.
Continue to Day 3 - OPS and OPS+
The careful reader may note that this means it’s not a “percentage” in any meaningful way. It’s certainly mathematically possible for someone’s slugging percentage to be above 1. It’s more accurately a “slugging rate”. ↩︎
This is when Babe Ruth discovered that home runs are pretty fun to hit, and players largely stopped treating baseball like a game of cricket. ↩︎
As a final note, it’s always funny to me how home runs are included in OBP. Of course it would be silly to not, but the name makes it a bit confusing. You don’t get on base with a home run. Maybe it should be the “Avoiding Outs Percentage” instead. ↩︎