April Homers

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.


April Temps and HRs

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.

Temperature Does Not Explain the Decline in April Home Runs
Temperature and League HR Splits
Steroids and Offensive Production by Zubin Jelveh

11 Responses “April Homers”

  1. ChuckO says:

    It’s already happened. On an recent ESPN broadcast, Twins vs. Red Sox, I believe, the announcers were talking about how homers were way down. They all agreed that it was due to testing.

  2. rubemode says:

    I’ll throw out this idea, your book makes the cases that expansion had a great impact on “offense”. One that I agree with. Could it now be the case that the league’s pitching has “caught up” and over come the watering down effects of expansion from the early and late 90′s?

  3. Donald A. Coffin says:

    FWIW, the decline appears to be entirely in the AL (see Joe Sheehan’s recent analysis of this at BaseballProspectus.com).

  4. Greyson says:

    I frequently wonder why there isn’t more time spent analyzing weather’s impact on the game. I think you can attribute weather as a large factor in the hot starts by both the Rays and Marlins this year, but no one seems to ever bring this up. (You can probably throw the D-Backs into the conversation too, but they have a much more talented team regardless.)

  5. Dan says:

    Very interesting, and I have to say the graph is extremely compelling. Thanks for taking the time to put this together.

    I know from a BP article that runs in the AL are down quite a bit more than in the NL (where they are flat from last year). Can you show the above graph split between the two leagues to see if the average temp for NL has been higher than AL? At the very least, it seems to me the best NL teams so far play in somewhat warmer climates than the best AL teams, but I could be wrong here.

  6. Chris says:

    When you did this analysis, did you just use the overall average temperature for the month, or did you go to the extent of analyzing the game-time temperature for each game?

  7. Andy says:

    I would also be interested to see the rate in domed stadiums over that time. I don’t know how possible it is, since some domes are retractable. Good anaylsis though!

  8. Adam says:

    The temperature graph is very startling in its correlation to homeruns. However, I’d be curious to know what theories there are for why 2002 stands out as the one year this wasn’t the case. Statistical anamoly is certainly an option, but I wonder if there was something else going on.

  9. Ken Houghton says:

    I’m inclined to suggest looking at, say, A-Rod’s April 2007 v. (A-Rod and Rodettes) in April 2008 and wonder if there are a few other outliers like that in the AL.

    But that still wouldn’t explain 2006.

  10. Steve says:

    Interesting, but the correlation isn’t as strong as it that graph makes it appear if you plot HR vs. temperature. The r-squared value is only about .35, and still only about .67 after dropping the two biggest outliers.

  11. Sal Paradise says:

    #4 Greyson:

    While I understand your thought process, if weather were to have that much of an impact, then you’d expect a better home record than away record.

    Marlins:
    Home: 13-11
    Away: 11-8

    Diamondbacks:
    Home: 19-8
    Away: 9-8

    Rays:
    Home: 16-8
    Away: 9-11

    Certainly true for the Diamondbacks and Rays.

    However, there’s also the matter of whether or not they are winning because of pitching, or because of hitting.

    Florida:
    Runs Scored: 5 (t-3rd in the NL)
    Runs Allowed: 4.8 (t-9th in the NL)

    Arizona:
    Runs Scored: 5.4 (2nd)
    Runs Allowed: 4.1 (2nd)

    Rays:
    Runs Scored: 4.6 (t-3rd)
    Runs Allowed: 4 (5th)

    The problem is that we would expect ‘hitter’s weather’ to help batters and hurt pitchers, which it doesn’t seem like is the case in Arizona’s case, or even the Rays case. And Florida doesn’t seem to have a good home-road split.

    There’s nothing exhaustive about this ‘research’ but it doesn’t seem to indicate it being a weather-related thing (though one would have to go deeper into temperature and weather and divide it that way to see if performances for these teams are correlated to temperature).