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Two years ago Mike Fast (now with the Houston Astros) took a look at pitchers who gained or lost velocity between the end of 2010 and spring 2011. We won't summarize his whole study here, but here’s the money quote:

There is definitely a predictive relationship between the fastball speed change observed in spring training and the fastball speed change observed in the regular season. Pitchers on average retained 41% of their spring training speed changes into the regular season, and the correlation coefficient for the sample here is r=0.53.

With that predictive relationship in mind, we've compared velocities from September 2012 and March 2013 and come up with some interesting results. This year's study includes World Baseball Classic pitchers, who were all limited in their usage by rule.

First, a summary of gainers and losers:

 Total Pitchers 69 Gainers 25 Losers 44 Average Change -0.4 mph
 MPH Change # of Pitchers 3 1 2 0 1 6 0 to 1 18 -1 to 0 24 -1 14 -2 5 -3 1

Most pitchers didn't change much at all, with 42 guys within one mile per hour of their September velocity one way or the other. There were more losers than gainers (44 to 25), with 20 guys losing at least one mph but only seven gaining that much.

The pitchers listed below each threw at least 20 fastballs or sinkers (combined) in both September 2012 and March 2013 as a starting pitcher. (Pitches thrown in relief by pitchers who also worked out of the bullpen were not considered.) The big table below shows the raw speed numbers and the change between the two periods. In addition, we reran the numbers using Mike's temperature adjustment technique (37 degrees = 1 mph fastball velocity change, baselining from 70 degrees) and recalculated the changes between periods. For sanity's sake we're showing only the raw numbers below, along with the “Delta” and “DeltaAdj”, the latter being based on the adjusted speed numbers.

 Pitcher SEP12 MAR13 Delta DeltaAdj Andrew Cashner 93.0 96.3 3.3 3.0 Patrick Corbin 91.3 93.2 2.0 1.7 Jarrod Parker 93.6 95.2 1.6 1.6 Homer Bailey 92.4 94.2 1.8 1.5 Kyle Kendrick 89.9 90.5 0.6 1.2 Luke Hochevar 93.1 94.3 1.2 1.1 Travis Wood 89.2 90.4 1.1 1.1 Jason Vargas 88.7 89.5 0.7 0.9 Madison Bumgarner 90.7 91.6 1.0 0.8 Jake Peavy 91.5 92.1 0.6 0.7 Ubaldo Jimenez 91.8 92.7 0.9 0.7 Tim Lincecum 90.4 91.0 0.6 0.6 Josh Beckett 91.5 91.6 0.1 0.6 Derek Holland 94.1 94.4 0.4 0.5 Edinson Volquez 93.8 93.9 0.1 0.4 Yu Darvish 93.9 94.2 0.3 0.3 Andy Pettitte 88.9 88.9 -0.1 0.3 Blake Beavan 90.8 91.3 0.6 0.3 Trevor Cahill 90.4 90.8 0.4 0.2 Bud Norris 92.2 92.3 0.2 0.2 Mike Fiers 88.9 89.1 0.3 0.2 Gio Gonzalez 93.5 93.7 0.2 0.1 Aaron Harang 90.2 90.4 0.2 0.1 Edwin Jackson 94.5 94.6 0.1 0.1 Ervin Santana 91.6 91.8 0.2 0.0 Mark Buehrle 85.6 84.9 -0.7 0.0 Ian Kennedy 90.2 90.1 -0.1 -0.1 Chris Sale 91.2 91.3 0.1 -0.1 Marco Estrada 90.7 90.4 -0.3 -0.2 Tommy Hanson 89.9 89.4 -0.5 -0.2 Mat Latos 93.2 92.8 -0.4 -0.3 Josh Johnson 93.5 92.6 -1.0 -0.3 Zack Greinke 92.9 92.5 -0.4 -0.4 Wandy Rodriguez 90.4 90.2 -0.3 -0.4 Matt Harrison 92.8 92.4 -0.4 -0.4 Barry Zito 84.5 84.1 -0.4 -0.6 Clayton Kershaw 93.8 93.3 -0.4 -0.6 Eric Stults 88.2 87.5 -0.7 -0.6 William Smith 90.3 90.1 -0.2 -0.7 Bronson Arroyo 87.9 87.4 -0.5 -0.7 Dan Straily 92.0 90.9 -1.0 -0.7 Hector Santiago 93.8 93.0 -0.8 -0.7 Jeff Samardzija 96.2 95.6 -0.5 -0.8 James Shields 93.8 93.4 -0.5 -0.8 Yovani Gallardo 92.4 91.4 -1.0 -0.9 Hisashi Iwakuma 90.6 90.1 -0.5 -0.9 Jordan Zimmermann 94.9 93.3 -1.6 -0.9 Scott Feldman 92.1 91.0 -1.0 -1.0 Tommy Milone 88.8 87.5 -1.2 -1.0 Chris Capuano 89.0 87.9 -1.1 -1.2 Wade Miley 91.2 90.1 -1.1 -1.2 Luis Mendoza 92.7 91.8 -1.0 -1.2 Samuel Deduno 91.7 90.2 -1.5 -1.2 Cliff Lee 92.3 90.2 -2.0 -1.3 Clayton Richard 90.7 89.6 -1.1 -1.3 Zach Stewart 89.5 88.6 -0.9 -1.3 Jerome Williams 93.7 92.5 -1.2 -1.4 Tyler Chatwood 94.4 93.1 -1.3 -1.4 Matt Moore 94.2 92.5 -1.7 -1.6 A.J. Griffin 90.5 88.8 -1.7 -1.6 Jeremy Guthrie 93.3 91.9 -1.5 -1.7 Joe Saunders 88.6 87.2 -1.4 -1.8 Doug Fister 90.0 88.2 -1.9 -1.9 Joe Blanton 91.2 89.0 -2.2 -2.0 Bruce Chen 87.2 85.1 -2.1 -2.1 Felix Hernandez 93.6 91.7 -1.9 -2.2 Justin Masterson 94.1 91.7 -2.4 -2.6 Wily Peralta 96.3 93.4 -2.9 -2.8 Ryan Vogelsong 92.7 89.5 -3.2 -3.2

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BuzzingThalami
4/01
Good stuff! Wish there were data on more pitchers. Would love to see Halladay's, for instance.