Saturday, March 07, 2009

The "Verducci Effect" revisited

The Wall Street Journal's new sports blog, "The Daily Fix," has revisited the "Verducci Effect."

That's the forecasting principle, invented by Sports Illustrated writer Tom Verducci, that when a young pitcher throws 30 more innings than in his previous season, he's due for a comedown next season.

But as I said before, I think the effect is simply regression to the mean. When a pitcher throws more innings than before, it's usually because he had a better year (since they don't normally let lousy pitchers throw a lot of innings). And when a pitcher has a better year, it's usually because he's somewhat lucky. And so he'll slide back to his normal level of performance the next year.

While I have no argument with the truth of Verducci's finding, I think it's not a matter of the innings, but, rather, a matter of the good performance.

In fact, I think that all things being equal, a pitcher with more innings is LESS likely to regress. Consider two 23-year-old pitchers: each has a career ERA of 4.50, and each pitched 100 innings in 2007. In 2008, both pitchers improved to 4.00. But pitcher A threw 105 innings, and pitcher B threw 150 innings.

According to Verducci, pitcher B is due for a comedown, while pitcher A is not. I disagree. I think pitcher A is more likely to drop back to his 4.50 career average. That's because there's less luck in pitcher B's record, and so his improvement from 4.50 to 4.00 is more likely to have been real.

I could be wrong.

P.S. Here's a piece by David Gassko, who did a control-group study and found no Verducci effect.

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At Saturday, March 07, 2009 9:53:00 PM, Blogger Brian Burke said...

How dare you question the Curse of 30! :)

At Sunday, March 08, 2009 9:23:00 PM, Anonymous Anonymous said...

I thought the Verducci Effect had more to do with probability of injury than with regression to the mean? I guess pitchers who jump 30 innings are more likely to have been injured the year beforehand and therefore more likely to get injuries than someone who had not been injured two years beforehand.

Gassko's article doesn't really make sense to me. He didn't include minor league innings, but somehow claims that will cancel out? The group with the "30-inning jump" probably is full of guys who did not actually have a 30-inning jump but just had some innings in the minors the first of the two years and Gassko isn't counting that.

At Monday, March 09, 2009 10:09:00 PM, Blogger Don Coffin said...

It does becomd an empirical question, in the end, doesn't it? I know the Baseball Prospectus folks have long tracked what they call "Pitcher Abuse Points" (and changed how they measure them as well). So who out there has done/might do such a study? (Not me.)


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