Research preview · declared rule regimes

Eras and rule-regime breaks

Baseball changes its own rules, and a metric fitted across a break measures two different games. These are the breaks this project declares, what each one invalidates, and which seasons may therefore be pooled. A model that crosses a break without saying so is not a longer-run model — it is a wrong one.

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Declared breaks

BreakSeasonWhat it changesInvalidates
Unavailable.

What this means for pooling

Every model in this project declares the seasons it was fitted on and the regime those seasons belong to. Where a model pools across a break, it says so and states why the pooled quantity is still meaningful. Where it cannot, the model is fitted per regime instead.

The 2026 automated ball-strike regime is a declared coordinate break for any pitch-level metric. No pitch-level metric is published, and the seam that would consume one validates the regime in both directions before admitting a single row — see Pitch data.