We’ve done pop-ups here before, but let’s really do them this time, right? I mean, let’s get it right this time at least. Because last time I was writing about IFFB%, which I thought was infield fly ball percentage. Turns out that’s infield flies divided by fly balls. That’s a little strange.
Steve Staude on FanGraphs is a proponent of infield fly balls divided by balls in play. Freed from the shackles of fly balls, we can get a sense of the pop up as a sustainable skill — IFFB% only has a .37 year-to-year correlation, but pop up percentage (PU%) is better, around .63. That’s better than the year-to-year correlation on home runs (.41)! We have a stat — FIP — that treats home runs as a skill that’s wholely under the control of the pitcher, and yet infield pop-ups are better correlated season to season.
Staude’s excellent initial work on the subject is not incredibly fantasy-relevant, but it does go some distance towards explaining park effects better. Look at the parks that showed a PU% that was more than .5% higher than IFFB% — Anaheim, Wrigley, Citi, Tampa, Detroit — and you get a list of the ‘secret’ pitcher’s parks. Of course, Milwaukee, Detroit and Yankee Stadium are on that list, too.
