The Inherent Bias Of The MLB Playoffs Run Environment
Image credit: (Photo by Bob Levey/Getty Images)
You’ve heard the axioms.
Pitching wins championships.
Runs are scarce in the postseason, so teams have to be able to win the close games in October.
In fact, it is generally true that the rate of runs scored in the postseason is lower than in the regular season. This makes intuitive sense in light of the best pitchers pitching the most important innings in October, when No. 5 starters become long relievers and the least effective relievers are minimized.
In the 28 completed MLB postseasons from 1995 to 2022, about 11% fewer runs were scored in October when compared with the regular season.
But this figure is by no means static from year to year. And in four postseasons—2002, 2004, 2011 and 2015—more runs were scored in the postseason than the regular season.
The graph above highlights how the postseason run environment is generally lower than the regular season environment, but that it bounces around unpredictably from year to year.
This volatility may seem confusing at first but make perfect sense when you think about it.
Firstly, almost all baseball data is volatile in small samples. The postseason run environment is clearly no exception to this rule.
But there’s a larger effect at play: survivorship bias.
The World Series participants influence the final postseason numbers to an outsized degree when compared with the regular season.
Over the course of a 162-game season, each individual team contributes almost exactly the same share—about 3.3%—to the collective. In the regular season, a 100-win team’s numbers are given one-thirtieth weight toward the overall MLB average. The same is true of a 100-loss team. The two teams effectively cancel each other out.
In the postseason, defeated teams stop contributing to the overall average. The end result is that the postseason average will take on the characteristics of the finalists, especially the World Series champion after it has dictated run environment terms in three individual postseason series.
The past two World Series champions illustrate this effect.
The 2022 Astros pitched to an American League-best 2.90 ERA in the regular season. Using only their best pitchers in October—highlighted by starters Justin Verlander and Framber Valdez and relievers Ryan Pressly, Rafael Montero and Bryan Abreu—Houston compiled a 2.29 postseason ERA and won the World Series.
As a result of the Astros’ pitching dominance, the 2022 postseason saw 17% fewer runs than the 2022 regular season. Incidentally, Astros pitchers contributed 17% of all innings to the 2022 postseason pie.
The 2021 Braves ranked third in the National League with 4.91 runs scored per game. Atlanta hitters took 21% of all postseason plate appearances that October as they marched to a championship. The Braves’ lineup was anchored by the likes of Freddie Freeman, Ozzie Albies, Austin Riley and World Series MVP Jorge Soler.
The American League pennant-winning 2021 Astros led the American League with 5.33 runs per game. Houston hitters took 22% of all postseason PAs that October, led by the likes of Jose Altuve, Yordan Alvarez, Alex Bregman, Carlos Correa and Kyle Tucker.
The result of the Braves and Astros taking 43% of all 2021 postseason plate appearances boosted the run environment that October. It was just 6% lower than it was during the regular season—or about half of the typical measured postseason effect.
It turns out that it’s a different axiom that fits best when comparing the contexts of the regular season and postseason.
When it comes to postseason run environment, history is written by the victors.
