Originally posted on Baseball Professor  |  Last updated 1/18/13
I spend a lot of time looking at pitchers’ repertoires on PitchFX. Part of that is the Moneyball effect, that looking at stats and numbers independent of names and scouting reports gives you valuable objectivity. The other part of that is there’s only so many hours in the day and I fill most of them with work, family, and other responsibilities. As much as I’d love to sit down and spend hours evaluating Wily Peralta, I have other things to do. And that’s why I love PitchFX. You lose a lot of the mental side of pitching — approaching batters, holding runners on, pitching with a lead, etc. — but you still get a very good feel for how a pitcher actually pitches and how good his stuff is. In my years of explaining why some pitchers succeed and why some do not, I’ve come across one fairly universal rule: The separation in velocity between a pitcher’s fastball and their change-up is extremely important. Francisco Liriano and Javier Vazquez have had some famously erratic seasons, and much of their success and failure correlates with how large the gap is between their four-seamer and their off-speed stuff. The idea behind this post is to decipher exactly what the relationship is between a pitcher’s fastball velocity, change-up velocity, ERA, and strikeout rate. I looked at data from 2007-2012 for all pitchers who had thrown at least 150 innings in a season. There were 589 such player-seasons, which were broken up into two groups: Pitchers who threw their change-up between 10-20% of the time (240 cases) Pitchers who threw their change-up at least 20% of the time (93 cases) This allows us to look at both pitchers who mix in a change-up as well as those who feature it. The data is displayed below. The number in the first column of each box is how many pitchers fell into that group. Threw change-up between 10-20% In this group of occasional change-up throwers, we see names such as David Price, Matt Cain, CC Sabathia, Cliff Lee, Wandy Rodriguez, Doug Fister, and Jake Westbrook. This group saw a strong inverse correlation between their fastball-change velocity gap and their ERA-. As this gap widened, their ERA- decreased. Because ERA- is a difficult stat to interpret on the fly, I also included what ERA this would have resulted in during 2012′s MLB climate (4.01 MLB average ERA). Remember, this analysis doesn’t at all speak to the quality of the rest of the pitchers’ repertoires, but all things equal it seems that even a three-mile-per-hour increase in average velocity gap from seven mph to 10 mph results in an ERA drop of almost one-third of a run. And aside from a blip in the 9.0 – 9.9 mph group, we see the strikeout rate rise in accordance with the velocity gap. Threw change-up at least 20% This data looks at only the players who feature their change-up, a group of players that includes names like Justin Verlander, Cole Hamels, Jeremy Hellickson, Mark Buehrle, Chris Capuano, James Shields, and Johan Santana. While the sample size in this group is much smaller, less than half of the previous group, the results are as we’d expect. The ERA- for the
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