Here’s something I’ve never understood:

This is the average innings pitched by the starting pitcher, by year, broken down by leagues.

There are at least two things about this plot that make no sense to me. Click through for more.

Mystery #1:

Forgetting the issue of the DH, I am perplexed why there is such a strong correlation between the two leagues. As you can see, with the exception of what happened in 1974 and 1975, the two leagues are strongly linked in terms of year-to-year change. Even without a statistics degree, I know this is not random chance. I would expect, however, that for what is a largely random grouping of teams, there would be noise in the data–for example based on specific pitchers and what kind of year they had on a team-by-team basis. Instead, we are seeing MLB-wide variations that are clearly affecting both leagues similarly every year. Given the up-and-down fluctuations of these systematic changes, the only solid theory I have is that it was the weather. We know that average yearly temperatures can vary by several degrees, and I can imagine this having a systematic effect on the tiring of pitchers across the country. (I have not been able to locate historical weather data–if you can, I’d love to get access to it to see how it correlates with the plot above.)

Mystery #2:

Logic suggests that the introduction of the DH would allow starting pitchers to go deeper into games. As we still see today in the NL, managers occasionally lift their starter a bit early in favor of a pinch-hitter at a key offensive moment of the game. In theory, the introduction of the DH in the AL in 1973 would have eliminated such choice, allowing managers to leave their starter in based only on his pitching performance. However, what we see is not a rise in the AL, but rather a drop in the NL.

At first, this seems really weird. The NL had no rules change in 1973, and so why would it be dramatically affected by a rules change in the other league? And why didn’t starting pitching duration change in the AL in 1973 with the introduction of such a radical rules change?

The lack of change in the American League can be explained by a few possibilities. Notably, scoring went up dramatically in 1973 (as one would expect by replacing pitchers in the lineup with a bona fide hitter.) Therefore, this may have tended to drive down starting pitcher duration based only on pitcher performance. The notion that starters were no longer removed for pinch-hitters may not have been the dominant factor.

It’s also true, though, that scoring in 1973 went up in the NL as well. This wasn’t due to the DH but was a “regular” fluctuation, perhaps related to the weather. It may be that the graph above shows us just what our intuition expects, but that it’s overlaid on an overall downward trend. In other words, in the mid 1970s, offense increased, starting pitcher duration shortened, but not as much in the AL because of the elimination of starting pitchers being removed for pinch hitters. So instead of seeing the NL stay the same and the AL go up, we see the NL drop and the AL stay the same.

I can imagine some other less likely possibilities having to do with the manager philosophy over starting and relief pitcher usage, but I won’t spell those out in detail here.