Studeman: 1947, the year baseball changed (but not for the reasons you think)

From SABR member Dave Studeman at The Hardball Times on August 27, 2013:

Here’s a graph of baseball data. There are three sets of data: the red circles occurred in the years 1902-1919; the black diamonds occurred in the years 1920-1946; and the yellow squares occurred in the years 1947-2012. All years are separated into the two leagues, so there are twice as many symbols as years. As you can see, the data sets are almost completely separate—there are no black diamonds hiding behind the yellow squares and only one black diamond behind the red circles.


What you see here are three changes in the fundamental relationship between slugging percentage (the bottom, horizontal, “x” axis) and the extent to which home runs drive slugging percentage (technically, home run bases as a percent of total bases, on the side, vertical, “y” axis). Before 1920, slugging percentages and home run rates were low, though there was a fairly strong relationship between the two. When slugging percentage went up, so did the totals bases contributed by home runs. (I’m trying to avoid cause-and-effect language here, but obviously home runs cause an increase in slugging—not vice-versa).

Between 1920 and 1946, slugging percentage and home run rates were much higher than in the previous era (all the data points are up and to the right), but the relationship between the two was weaker. Take a close look: the points are more spread out and the slope is flatter. During this period, batters increased their slugging percentage in somewhat less homer-centric ways. They hit more singles, doubles and triples. They were more versatile sluggers.

From 1947 on, however, increases in slugging percentages were closely associated with increases in home runs. Look again: the yellow squares are much closer together than in the second era, and the slope is steepest of all. And this is key, too: the overall range of slugging percentage was roughly the same in the second and third eras; it was the role of the home run that changed.

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Originally published: August 27, 2013. Last Updated: August 27, 2013.