For the second year in a row, home runs are down in baseball. It won’t be long until someone attributes this change to improved drug testing and the response to the the Mitchell Report. Here is a graph of March/April home runs (thanks to Baseball-Reference’s league splits).
Now, it is possible that testing might have had an effect, but it is not obvious in the data. MLB began random tests with sanctions for a first offense in 2005. Home runs were down in 2005, but they were not significantly different from prior years; and, in 2006 home runs returned to the 2004 level.
But still, there is no denying that home runs are down from recent historical highs. In fact, the home-run rate this year is the lowest it has been since 1993—the beginning of the modern home-run era. And this continues the decline from last season, when the home-run rate dropped below three percent for the first time since 1997. Are there any factors that could be causing this aside from a decline in drug use or random variation in the data?
It is easy to remember something very unique about last season’s first few weeks: it was unseasonably cool. Off the top of my head, I recall the Indians losing several games to snow and playing a home series with the Angels in Milwaukee. Chris Constancio looked at the impact of temperature on home runs and found strong positive relationship the two.
The average temperature for major league baseball games [in 2007], 58.2 degrees Farenheit, was over four degrees cooler than the average during the early part of the previous two seasons. This relationship seems relevant to understanding why home runs, and consequently run totals, are down this season….
Game-time temperature was a significant predictor (at the p [greater than] .001 level) of whether or not batted balls left the ballpark, but the day of the season was not statistically significant. A batted ball has a 4.0% chance of leaving the park during a game played in 70 degree conditions, but only a 3.5% chance of becoming a home run in a game played in 50 degree conditions. This relationship exists regardless of whether or not the game is being played during the first week of the season or in the middle of May.
In summary, it’s true that hitters gain an advantage in hitting home runs as the season progresses, but this advantage can be explained entirely by accounting for air temperature changes.
This year, I can only recall one snow-out (Braves–Rockies), and the early-season temperature hasn’t gotten as much press as it did last year. I decided to look at how the April temperature this season differs from previous seasons, and compare temperatures to home-run rates.
Over the past decade, early-season home runs and temperature have moved together. April 2008 was actually 0.3 degrees cooler than April 2007—sorry Al Gore*—and 5.12 degrees cooler than April 2006. This doesn’t mean drug testing has not affected hitting power, but we have have a decent alternative explanation for why home runs are down. So, make sure you point this out to the first person who claims drug testing is working.
*It’s a joke, no angry e-mails please.
UPDATE: Upon closer inspection, it appears that temperature changes do not appear to explain much of the decline in home runs. See the following links.