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Why Are so Many Runs Scored in the Bottom of the First Inning?

After starting to look at some inning-by-inning data from my baseball win expectancy finder for another project, I stumbled across something weird that I can’t explain. Here’s a graph of expected runs scored per inning:

Graph of expected runs by inning

Check out how high the bottom of the first inning is. On average, 0.6 runs are scored then compared to 0.5 runs in the top of the first. That’s a huge difference! Let’s look closer:

Graph of home advantage in runs by inning

Holy outlier, Batman! So what’s going on? Here are some ideas:

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