In light of yesterday’s article on Infield Fly Balls and xFIP, there were some questions and debate about if popups are something under a pitcher’s control, or what you might call a skill. After reading the comments, I was somewhat doubting that my research might not have been thorough enough to essentially rule them out as a skill (which is what I more or less did).
If you look at popups per ball in play on a year-to-year basis, you get a correlation of about .52, which would highly suggest that there is some “skill” in inducing popups. However, there is a very strong positive correlation between popups and outfield-fly-balls (.64), and a very strong negative correlation between popups and groundballs (-.72).
In other words, as outfield-fly-balls increase, so do popups. As groundballs decrease, popups increase. For comparisons sake, line-drives have absolutely no correlation with popups.
Since the correlations are so high, you can basically come up with an expected popup rate based on a player’s groundball percentage. To me, it actually looks non-linear:
So it seems that each player has a dynamic expected popup rate based on his groundball percentage. Now the real question is, do players popup rates diverge from their expected popup rates consistently on a year to year basis?
If you look at the above chart, you’ll see that there’s not much consistency from year-to-year. The correlation is about .18, which pretty much agrees with Mitchel Lichtman’s findings of .14 as quoted in David Gassko’s Batted Ball DIPS article. For comparisons sake, BABIP has a year-to-year correlation of .15.
So what does this mean for popups as a “skill”? I’d say they are sort of a skill that is closely tied to groundball percentage, but from the findings above, that’s about as far as I’d go. While there may be certain pitchers that prove to be exceptions (just as there are exceptions with BABIP), popups in general do not seem to be much of an independent skill.
All batted ball data is from Baseball Info Solutions from 2006-2009 for pitchers with at least 100 innings pitched.
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