Method · expected_woba_gap@v1

Expected wOBA gap

The distance between what a hitter actually produced and what Statcast expected from his contact quality. It separates a real decline from a run of bad luck: if the expected number falls with the actual one, the hitting got worse. If only the actual one falls, the outcomes did.

The filter, exactly as it runs

woba IS NOT NULL AND est_woba IS NOT NULL

Where this is arguable

Expected wOBA is built from exit velocity and launch angle and assumes league-average defense and park. A hitter who plays half his games somewhere unusual is described less well by it.
It cannot see approach. A hitter who stops swinging at strikes shows up as worse contact rather than as a worse decision, which is a different problem with a different fix.
Two seasons of agreement is suggestive, not proof. Contact quality itself carries year-to-year noise.

These are the objections I already know about. If you have a better one, or you think a different threshold tells a truer story, say so and I will re-run it with your numbers and publish what comes back.

Version 1. Older versions are never deleted, so a piece that citesexpected_woba_gap@v1 stays reproducible after this definition is revised. Source: Data: nflverse (nflfastR), CC-BY 4.0