xFIP is a refinement of FIP that replaces the pitcher’s actual home runs allowed with what they would have allowed at a league-average HR/FB rate. The thinking is that fly balls are mostly a stable skill, but the percentage of fly balls that leave the park bounces around year to year.

A pitcher with a very low FIP but a very high xFIP is probably benefiting from suppressed HR/FB — he’s allowing flyballs but few are leaving the yard, and that’s unlikely to last. The reverse is also true: a pitcher with high FIP but low xFIP has been giving up homers at an unsustainable rate.

xFIP is most useful as a regression signal. When FIP and xFIP diverge meaningfully, the gap usually narrows over time. NUT relies on FIP rather than xFIP because FIP captures actual pitcher value (homers happened, runs scored), while xFIP captures expected value going forward.

What is a good xFIP?

xFIP is a stricter version of FIP that strips out home-run-rate variance. NUT uses regular FIP, but xFIP appears in the pitcher Value percentile section as a sanity check on whether a pitcher’s FIP is being inflated or deflated by an unsustainable HR/FB rate.

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