Are Popups a “Skill”?  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