Two recent threads have brought up the appropriateness of using xFIP to judge past performance for pitchers. (And Adam's tie-in to his career touches on a related point, too.) I'll summarize the arguments for and against using xFIP as a retrospective metric below, but the reason I bring this up is because I think the arguments are more nuanced than these simplified viewpoints: Against using xFIP as a retrospective measure: Sure, HR/FB might need a large sample size to become a good representation of a pitcher's skill, but those home runs actually happened and the pitcher is the one responsible for them happening. For a historical metric, we should use actual home runs, not a home run estimate based on fly balls. (One could also argue that any sort of FIP measure removes a pitcher's performance with runners on base, which is not right, because that "clutchiness" performance actually happened, too.) For using xFIP as a retrospective measure: Yes, those home runs occurred when the pitcher was on the mound, but, like BABIP, they aren't necessarily a result of his talent. The cliche is that "pitchers allow fly balls, but hitters turn them into home runs." If we're going to assess a pitcher's future HR skill based on FB's, then why would measuring his past skill be any different? I see two levels of discussion here: which of those two simplified arguments do you agree with (and why) and what nuances do they miss that are important (and why)? For example, are there more distinctions to be made than just past/future? Where does the distinction between past value and past talent come into play?