Last month, I wrote about the two leagues, noting that

1. The American League, perceived as being bad this year, was actually a good deal better than the National League overall, and
2. The perception of the American League’s weakness was due to a near-record level of parity, with neither great nor bad teams.

Let’s start with the second point. At the time of the post, through games of September 5, the standard deviation of winning percentages among American League clubs was the lowest it has been in the 30-team era. Projected onto a 162-game season, the standard deviation of wins for American League teams was 7.8, barely eking out 2007’s 7.9 as the most egalitarian distribution of wins since 1998.

Since September 5, a .500 record has become a black hole, exerting irresistible gravity throughout the American League galaxy:

• Of the teams with the six best records in the league on that date–the Royals, Blue Jays, Yankees, Astros, Rangers, and Twins–only Toronto and Texas had a winning record the rest of the season.
• Baltimore, the sixth-worst team in the league as of the morning of September 6, tied the Jays for the best record in the East thereafter. Boston, then the third-worst team, went 15-12 the rest of the way.
• Cleveland, four games below .500 at the time, scrambled to finish 81-80.

Overall, parity in the already-equality-loving Junior Circuit increased, by so much that I looked beyond the post-1998 30-team era. I calculated the standard deviation of winning percentages for every league-season since 1901. I then multiplied the standard deviations by 162 to arrive at the standard deviation of wins over a 162-game season. Yes, I know, most of those seasons were shorter than 162 games, but that’s OK; I’m just looking to turn the standard deviation of winning percentages, which is not an intuitive figure (e.g., American League, 1930, 0.1107), into something that is recognizable (17.9 wins). Here are the ten seasons in baseball history with the highest parity, that is, the lowest standard deviation of wins:

The 2015 American League is the most egalitarian, populist, tax the rich/feed the poor, Kumbaya-singing league in baseball history. As I suggested in September, it’s the Sweden of leagues.

(The National League finished 2015 with a standard deviation of 13.1 wins, ranking it 102 out of 230 league-seasons in terms of parity. It was the ninth-most unequal among 36 league-seasons since the expansion to 30 teams in 1998. For Gini coefficient detractors, the most unequal league ever was the 1909 National League, which featured the 110-42 Pirates, 104-49 Cubs, and 92-61 Giants, along with the 55-98 Dodgers, 54-98 Cardinals (Yadi was hurt), and 45-108 Braves.

Now, as to the other point, the American League’s superiority over the National League despite its group hug ethic, here’s a chart.

Twelve years and running.