Limitations

Read this before quoting any number from this project.

Nothing is validated yet

The project is pre-release. No results have been published. The claim under test has not been run.

Defence remains hard

Box-score defensive statistics capture a fraction of defensive impact. Steals and blocks are visible; positioning, communication, and deterrence are not. Metrics built only on box-score defence are unreliable, which is why this project excludes DBPM outright and leans on plus-minus based estimation for the defensive side.

This is a limitation of the available data, not something a better model fixes.

Low-minute players

Every impact metric is noisy below roughly 500 minutes. This project does not solve that. It does two honest things instead: it shrinks such players toward the league mean, and it reports an interval wide enough to show that the estimate is weak.

A wide interval is not a defect. It is the answer.

Era dependence

Reliabilities are measured on modern data. Applying them to the 1990s, before three-point volume and tracking data, is unsupported. Play-by-play coverage itself only reaches back to about 2002 in the bulk sources used here.

Correlated errors between metrics

Inverse-variance weighting assumes independent errors. Public metrics share inputs, so they are not independent. The model estimates a correlation structure rather than assuming it away, but the assumption is imperfect and the residual effect is to make intervals slightly too narrow.

RAPM does not establish causation

RAPM measures association between a player being on the floor and the score moving. Coaches choose lineups for reasons, and those reasons correlate with outcomes. A player who only plays against weak opposition will look better than they are, beyond what opponent adjustment removes.

What this project cannot tell you

• Whether a player will be good on a different team, in a different role.
• Whether a player's improvement is real or noise, within a single season.
• Anything about players outside the NBA.
• Anything useful about betting, which is out of scope by choice.