Note: This post has been posted with permission from here.
With Biogenesis, A-Rod, and steroids all over the news lately, it brings up the age old question: what is the effect of steroids on athlete performance? David Schoenfield, in this ESPN.com article, argues that steroids usage didn’t drive performance in the early 2000’s. Simply, today’s hitters just aren’t making enough contact to put up big numbers. He claims that hitters are hitting home runs at a similar rate when accounting for the increased strikeout rate and bigger strike zones called in today’s game. I was skeptical, and I decided to put the numbers to the test.
Schoenfield’s argument rests on the fact that home run rate per contact, the percentage of balls put in play that result in a home run, has stayed nearly constant from 1993 to 2013. He cites a home run per contact rate of 3.13% in 1993, a peak of 4.19% in 2000, and a rate of 3.68% in 2013. A deviation of about one percentage over an entire season, nothing to see here right?
But wait, Schoenfield doesn’t include the sample size that this deviation occurred over. Using Baseball Almanac’s season-by-season league batting averages, league home run totals, and league strikeout totals, I reconstructed his statistics and decided to compare the home run rate in 2000, the height of the Steroid Era, versus the home run rate in 2012, accepted to be past the end of the Steroid Era. Using the fact that an entire year of at-bats as an independent sample and home run as a nominal/binary outcome, I set up a two sample z-test for proportions to see if there was a statistically significant difference in these rates.
Our default, or null, hypothesis is that these rates do not differ, while the alternative is that they do. First, let’s look at all at-bats. Home runs were hit on 3.04% of all at-bats in 2000 and on 2.98% of all at-bats in 2012, and we can reject our null hypothesis and conclude these rates are statistically significant. Home runs were hit at a higher rate in 2000 than in 2012, lending credit to the effects of the Steroid Era. But what if we do what Schoenfield suggests and remove strike outs? Removing strike outs drops the sample size of at-bats from 167,290 to 135,934 in 2000 and from 165,251 to 128,825 in 2012. Yet, home runs were hit on 4.19% of balls put in play in 2000 versus 3.83% of balls put in play in 2012. This, these rates are statistically significant as well, disproving the strike outs theory.
Yes, strike outs rose from 19% of at-bats in 2000 to 22% in 2012, but this doesn’t explain the change in home run rate that occurred in the Steroid Era. Steroids may not teach you how to hit a baseball, but they sure can help you hit it farther.