Mains: Groundballs through the decades

From SABR member Rob Mains at Baseball Prospectus on June 22, 2016:

I am a Bill James fan. You might not be. That’s okay. For my money, if there’s no Bill James, I’m probably not writing this.

So it caused some dissonance on my part when I wrote that I thought James’ antipathy for groundball pitchers is misplaced. One of my general rules in life is that if you come to a conclusion that’s seriously at odds with conventional thinking, you could well be right, but to be safe, be sure you can answer in the negative the question Am I doing something stupid?

I really don’t think I am. I found that groundball pitchers may—may—allow a few more walks, and they get a handful fewer strikeouts, but by allowing a lot fewer homers, they allow fewer runs. That’s true in terms of both outcomes (ERA) and process (FIP).


I ranked every 2015 pitcher by groundball rate, and divided them into 10 equal-sized deciles based on plate appearances. As you can see, the pitchers who yielded the most grounders on batted balls averaged 61.8 percent grounders. They gave up homers in 1.9 percent of plate appearances, compiling an aggregate ERA of 3.47 and FIP of 3.62. By contrast, the pitchers who gave up the fewest grounders, averaging 33.5 percent, had a 3.6 percent home run rate, a 4.22 ERA, and a 4.43 FIP. That’s pretty clear, right? And those aren’t cherry-picked numbers. ERA, FIP, and home run rate are all negatively correlated to groundball rate, as the last line shows. The more grounders, the fewer homers and the lower ERA and FIP. The regression relationship is this: For every 10 percentage point increase in groundball rate, pitchers reduce their ERA by 0.28, FIP by 0.27, and home run percentage by 0.006 percent. So James is wrong when he says, “If you like Ground Ball pitchers, you’re welcome to them.” Isn’t he?

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Originally published: June 22, 2016. Last Updated: June 22, 2016.