A baseball blogger points out that home runs per at bat in April decreased for the second consecutive year.
The reasons for the decrease aren’t as obvious as you might expect.
Blogger JC over at SABERNOMICS pulled the numbers from BASEBALL-REFERENCE and did the math. While all of us are thinking, “it’s the steriods, stupid,” or lack thereof because of testing, but JC says that may not be so.
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.
JC quotes Chris Constancio of HARDBALL TIMES, who suggests the drop might correlate to a decrease of another sort:
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…of whether or not batted balls left the ballpark, but the day of the season was not statistically significant…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.
The correlation is staggering. Maybe ballparks would warm up if teams starting burning copies of the Mitchell Report along the warning track.