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Comforts of home

Rogers better at Comerica, but is the success legit?

Posted: Monday October 23, 2006 2:08AM; Updated: Monday October 23, 2006 3:57AM
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Kenny Rogers has had more strikeouts at home this season and less good contact when the batter puts the ball in play.
Kenny Rogers has had more strikeouts at home this season and less good contact when the batter puts the ball in play.
Tom Szczerbowski/US PRESSWIRE
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By Nate Silver, BaseballProspectus.com

DETROIT -- The conspiracy theory is as follows. Kenny Rogers is scuffing his pitches. But he only bothers to do so when he pitches at Comerica Park. That's why Jim Leyland has engineered his rotation throughout the postseason to ensure that Rogers pitches at home.

Let's think this through for a moment. Suppose that Rogers is cheating. Why would he only cheat at home? The usual ingredients that one uses to scuff a pitch are dirt, rosin and spit. Maybe pine tar. Last I checked, those items are as readily available in Yankee Stadium or the Network Associates Coliseum as they are in Detroit.

But let's put that aside for a moment and give the theory its day in court. If a pitcher were doctoring his pitches at home -- but not on the road -- what evidence would we expect to see? I'd think we'd expect to see two things: more strikeouts at home and less good contact when the batter does manage to put the ball into play. Let's examine the evidence on each of those things.

Strikeout Rate. At home this season, including his three postseason starts at Comerica, Rogers has struck out the batter in 13.9% of his plate appearances. On the road this season, Rogers has struck out the batter in 11.1% of his plate appearances.

• Good Contact. At home this season, excluding plate appearances that ended with a strikeout, walk or hit batsman, Rogers has allowed an extra-base hit 6.9% of the time. On the road this season, excluding plate appearances that ended with a strikeout, walk or hit batsman, Rogers has allowed an extra-base hit 12.2% of the time.

Rogers' walk rate is slightly worse at home than on the road (8.1% versus 6.6%). His groundball-to-flyball ratio is almost exactly the same at home and on the road. He gave up about the same number of singles at home and on the road. It's the strikeout rate and the "good contact" categories where we see the disparities in his splits.

These differences are right on the verge of what we data geeks like to call "statistically significant". In other words, we can conclude that Rogers strikeout rate, in all probability, really and truly is higher at Comerica Park, and his "good contact" rate really and truly is lower at Comerica Park. It isn't just a random fluke.

What we don't know is why Rogers' numbers are better at Comerica. Park effects are almost certainly part of it. Comerica reduces home runs by about 20% and doubles by about 8%. But the differences remain statistically significant even after adjusting for these effects.

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