Q: What does EPA/play actually tell us about QBs?

A: Mainly, it& #39;s saying how good they are at moving the chains, plus how often they& #39;re turning the ball over.

EPA adds context on top of those two things, but together they explain ~87% of a starting QB& #39;s season EPA/play.

Interestingly, these two stats—first down rate and turnover rate—aren& #39;t the most useful tools for projecting a QB& #39;s success going forward.

They& #39;re just quantifying such large-impact events that they explain a great deal of why a QB& #39;s plays had the overall effect they did.

I shouldn& #39;t equate them. A quarterback& #39;s first-down% explains 76% of in-sample EPA/play and predicts next-season EPA/play with an R^2 of 13%. Not bad!

Considered together, a QB& #39;s turnover rate and sack rate explain 38% of same-season EPA/play but predict only 3% of next year& #39;s.

A quarterback& #39;s sack rates predict themselves year-to-year with an R^2 of 20%. His INT rates predict themselves with an R^2 of 13%. (Fumbles lost being far more chaotic.) Neither one is very predictive of next-season EPA/play. https://twitter.com/Cooper_DFF/status/1265334939236675585">https://twitter.com/Cooper_DF...

Interestingly, if you want to predict a quarterback& #39;s first-down rate next season, you& #39;re a bit better off looking at his completion percentage over expectation (CPOE) than the actual rate at which he moves the chains.

CPOE is a pretty effective tool.