Are home runs more likely at the beginning of games when the air temperature is typically marginally warmer than at the end? That is the question that I recall in my head, but I am unsure if it drove me to split home run rates by inning or if it formed as a question that could possibly be addressed by data cross tabbed in that manner. Regardless of which came first, I did end up looking at home runs per inning, but I don’t think you can say much of anything about temperature changes during the game from this.
Here I charted the 2011 home run rate in each half inning against the rate for the team (either home or road) overall. Home teams enjoy a slight but meaningful advantage in hitting home runs and I wanted to normalize for that. In the above graph, the points at the second inning show that road teams are more likely to hit a home run in that inning relative to all other top half innings.
The dramatic difference between the top of the third and top of the fourth surprised me, especially since the home teams showed a much more gentle increase. In the other seven innings, the home and road squads track closely to each other, but not in those two innings. One year of data is hardly a definitive sample though and there’s no reason to stop there, so onward I went to the entire Retrosheet era.
That certainly got rid of any discrepancies between home and away, but the general pattern in the 2011 data held and just became more concrete. I expected the graph to look something like an arc and instead got something more like arches. The fourth and sixth innings are baseball’s home run innings and it’s not close. I internally guessed that starting pitcher fatigue would have the middle innings be the most prone for home but that’s also why I expected an arc-like curve descending as the relievers took over. What’s going on in the fifth? Batting order plays a prominent role, but whether that’s the whole story is worth a deeper look.