Possessions are cut into stints of unchanged personnel from GameRotation, then a ridge regression assigns each player an offensive and a defensive coefficient per 100 possessions. The penalty is cross-validated per season (lambda = 3200), and the fit shrinks toward a box-score prior rather than toward zero.
Shrinking a thin sample toward zero calls a 200-possession player exactly average. The prior is a fit on per-100 box-score rates, explaining about 34% of offensive and 10% of defensive variance. Each prediction is itself damped toward the fitted league mean by that player's own possession count, because an unshrunk prior becomes the estimate whenever the data is thin.
| Model | RMSE |
|---|---|
| RAPM | 15.17 |
| Home court only | 16.58 |
RAPM wins in all 6 seasons individually as well as pooled.
Intervals are 95% shrinkage-aware sandwich intervals, approximate rather than posterior: the penalty is plugged in from cross-validation rather than modelled with its own uncertainty. Players whose intervals overlap are grouped into tiers instead of ranked, because a 1-to-N list would assert distinctions the intervals do not support.
Lineup synergy, the residual between a five-man unit and the sum of its parts, does not persist out of sample (r = 0.02, n = 321), so it is published as description, not as a forecast. 190 of 7230 games are skipped where possession reconstruction does not tie to the official final.
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