2015: A Season of Unprecedented Parity In the American League

Background: When the 2015 season ended, I remarked to myself that there seemed to be a great amount of parity in the American League this year. So I decided to see whether it was just my faulty impression, or if it was indeed a closer race this year from years past.

Methodology: I decided to use variance in win percentages among teams in each season to define parity, with a lower variance equating to more parity.

Variance is a measure of the spread of a dataset. It is calculated as follows:
variance equation
where N=population size, mu = population mean, x_i = data entry.

I took my dataset from baseball-reference.com and used Python scripts to modify the raw data into a cleaner .csv format, so that I could run analysis in R.

The 2015 season had the lowest variance (.001836222) in win percentage of any season in the history of the American League (1901-2015).

Here is a time plot of the variances across seasons:
On the left, 0 is 2015 and it increases by one season as the graph goes to the right.

Conclusion: 2015 was in fact the season with the most parity all-time in the American League.

The American League season with the worst parity? Go back to 1932, where the Babe Ruth and Lou Gehrig-led Yankees won 107 games and the Boston Red Sox lost 111. (Variance = 0.01710932)

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Parity is the word Manfred and every other commissioner in the the past used when they wanted to sell you on an overall crappy product. Parity is nothing more than a fancy slap in the face for mediocrity. You can hide anything in mediocrity. The mental side of hitting needs to be worked on most, because swinging for the fences whether two strikes or zero strikes on the batter is a losing formula.