2022 Blogmas Day 11 – Earned Run Average

Let us leave the world of sluggers and slap-hitters behind us, and move 60 feet forward to the pitcher’s mound. I’ve always enjoyed and valued pitching over hitting in my baseball life. As such, the first ten days of hitting required me to learn more than I anticipated, while I feel more prepared to handle this section of Blogmas.¹ We’re not starting as simple as possible, but we are starting with the most important of the original pitching statistics: how good are you at preventing runs from scoring?

We cut pitchers a little slack and only pay attention to earned runs, which are runs scored without the benefit of a defensive error.² There is a certain art to deciding earned runs. What many people don’t know is that every MLB game has an official scorekeeper, who decides (among other things) whether a play was a hit or an error. As such, they become responsible for determining whether a run is earned or unearned. They do so be “reconstructing” the inning were the errors not to have happened; an error with 2 outs that lets the inning continue all but guarantees any further runs will be unearned.

All that is just some context and trivia. What matters is that a pitcher is charged earned runs (ER), which are any runs that score without any funny business going on. Earned Run Average (ERA) is how many earned runs they give up per nine innings: were they to pitch a complete game, how many runs would we expect the opposing team to score? We compute it by dividing their earned runs by innings pitched (IP), and multiplying by 9.

\text{ERA} = \frac{\text{ER}}{\text{IP}} \times 9

At a base level, it’s easier to attribute runs against a pitcher than it is to attribute them to a batter. So for a given season, ERA is an excellent and simple way to compare pitchers. Other statistics that we’ll cover later can be indicative of how they wound up with their ERA, but are less important overall.

Of course a few factors are being wholly ignored. While only considering earned runs “normalizes” for defense, a great defense can turn what would otherwise be a clean hit into an excellent out. If they can get to more balls, they can bolster a pitcher’s ERA.³ The catcher that is paired with the pitcher is also important, as are factors related to the ballpark. We’ll discuss those in a later post, but it’s worth getting out in front of what ERA misses.

Now for a bullet-point story about ERA.

The starting pitcher with the lowest ERA in a season since 1920⁴ is Bob Gibson, who had a 1.12 ERA in 1968.
To put that in perspective, the next-best was Dwight Gooden’s 1.53 ERA in 1985, which is 37% higher.
Between 1950 and 1968, all MLB mounds were set at 15 inches tall.⁵ Due partially to Gibson’s success — pitchers overall had a great year and 1968 is called “The Year of the Pitcher” — the mound was reduced to 10 inches, where it remains today.
Since the mound reduction, there have been 27 qualified starting pitcher seasons⁶ with an ERA below 2.00.
Only four players — Greg Maddux, Pedro Martinez, Roger Clemens, and Clayton Kershaw — have two sub-2.00 ERA seasons since 1969.

Continue to Day 12 – Other Basic Pitching Statistics

1
I’m still doing my research to make sure I have everything correct.
2
Here’s an edge case that I love. If there’s a runner on third base, and the ball gets past the catcher thus allowing the runner to score, is that an earned run? Well, it depends. If it is scored as a passed ball, which is how we say it was the catcher’s fault, then it is not an earned run. If it’s a wild pitch, which we say is solely the pitcher’s fault, then it is an earned run. The rule of thumb here is that if the ball hits the dirt before it reaches the catcher, then it’s a wild pitch.
3
The effect is not massive because even the worst MLB defenses aren’t that bad, but it’s worth noting.
4
There are only 3 better, in 1880, 1914, and 1906, so this restriction isn’t a big deal.
5
Before 1950, they just couldn’t be higher than 15 inches. Starting in 1950, they standardized on 15 for everyone, although back then teams were still a little cheeky and many were suspected of having a slightly higher mound still.
6
Similar to batting, you need to play enough. Qualification is 162 innings in a season.

Leave a Reply Cancel reply