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Friday, April 17, 2009
Is the ball juiced again?
Greg sent me an email:
Thought you might like to hear about something I’ve been following for the first week of the season. I began wondering at the large number of long home runs being hit in the first two full days of the season, and started watching the numbers closely. The distance of the home runs being hit this year (the true distance, i.e where they actually land, as well as the standard distance, which factors out weather and altitude) is significantly higher than last year, with the average standard distance being 8.5 feet longer this year than last.
You may be wondering about sample sizes, and of course I took that into account. I used a 2-sample T-test on the 2009 and 2008 full season data, and got this:
Two-Sample T-Test and CI: 2009, 2008
Two-sample T for 2009 vs 2008
N Mean StDev SE Mean
2009 199 399.8 27.8 2.0
2008 4820 391.3 25.4 0.37
Difference = mu (2009) - mu (2008)
Estimate for difference: 8.49
95% CI for difference: (4.54, 12.45)
T-Test of difference = 0 (vs not =)
: T-Value = 4.23 P-Value = 0.000 DF = 211
The p-value actually works out to 0.0000341, which is a very strong indicator that something is making 2009 home runs fly farther than 2008 home runs, in isolation of the weather, and to me that implicates the ball. In the course of observing all the homers, I have also heard lots of comments from announcers who were surprised at how far the ball had carried.
When I look at only April, 2008, I get a p-value of 0.01, so I don’t think it’s just some sort of calendar thing here. I’ve done the same comparison to 2007, 2006, and the month of April for each of those years, and all indications agree that the difference is significant.
So, you might want to dust off your calculations from that “Changes in HR Rates from the Retrosheet Years” article and see what you get. Looks like a big year for homers, and so far the actual rate of 2.14 HR per game (in April!) doesn’t contradict that…
I’ll only be able to report my results at the end of the year. Drastic single-year changes only happens when you have a catalyst, as discussed in my article on the subject.
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These are Greg’s images from post 48:
