The Parity Year

Here is my most recent FanGraphs piece. (It made The Best of FanGraphs list for last week!) It revisits the work I did in September on parity between the two leagues. At the time, I noted that the American League, perceived as being weak this year, in fact is overall better than the National League (as measured by interleague won-lost record). I attributed the perception of weakness to the near-record level of parity in the American League, with neither notably strong nor weak years. As measured by the standard deviation of wins, in fact, the American League in 2015 had, at the time, the highest level of parity in the 30-team (since 1998) era. 

With the regular season in the books, there are two conclusions. First, parity in the American League didn't turn out to be the highest of the 30-team era. It was the highest of all time. Here is a list of the standard deviation of wins, i.e., the "plus or minus x" amount in the phrase, "The average team had 81 wins, plus or minus x." Here's a list of the ten seasons with the lowest standard deviation of wins since 1901:

The American League this season was the most equal league in major league history.

Despite that parity, the American League clearly was the superior league, for the twelfth straight season: