"...your life will never be the same again: IFFB% may not mean what you think it does. Now, FB%, for example — that’s defined as fly balls divided by batted balls, right? Many of us might therefore assume that IFFB% equals infield fly balls divided by batted balls… but it doesn’t. IFFB% is actually infield fly balls divided by fly balls. This means that IFFB% doesn’t tell you much about a player unless you have the context of his FB% to go with it. It also means that IFFB% * FB% equals what you probably thought IFFB% was, which is IFFB/(Batted Balls). Hopefully you’ll be able to read this clearly through your tears: I’m going to introduce a new, not-officially-FanGraphs-sanctioned term here: IFFB% * FB% = PU%. PU%, or popup percentage — again, it’s what you probably thought IFFB% meant — is the percentage of batted balls that are infield flies. This leaves OFFB%, or outfield fly balls, as the remainder of FB% (i.e., FB% = PU%+OFFB%)."