Petti: Pitcher attrition and velocity decline by age

From SABR member Bill Petti at FanGraphs on June 6, 2012:

On May 28, the Twins released 33-year-old starter Jason Marquis. Considering how bad Twins pitchers have been this season, Marquis’ numbers stood out as being even worse.

In seven starts, Marquis posted a 8.47 ERA and a 7.25 FIP, numbers driven largely by a 7.5% strikeout rate, a 27.3% HR/FB and a sky-high .352 BABIP against. There was some speculation that, since Marquis has lost about 2 mph on his fastball since 2009, it was likely that he’d become more hittable — particularly at age 33.

This led to some discussion on Twitter about whether there was a greater likelihood that Marquis’ velocity drop at his age was more of an issue than if he had been a younger pitcher. It’s a legitimate question — whether diminished velocity has an impact, depending on age.

 The answer seems straight-forward: calculate the average performance for each age cohort in the season they lost at least 1 mph and for the season after they lost the velocity, then compare the two. Easy enough. But the answer is complicated because some pitchers will have left the sample by the following year.

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Originally published: June 6, 2012. Last Updated: June 6, 2012.