The highest-value adjuster and the thinnest sample in baseball. The regression
is not a refinement here, it is the entire feature: applying raw splits would
adjust projections on noise, which is worse than not building it.
PHASE 0 — both gates clear, and one was already closed. Splits are a statsapi
pull, one call per hitter (statSplits with sitCodes vl,vr). The batter-handedness
join that Session 69 recorded as pending is in fact DONE: statcast_aggregates
carries bats for 604 of 604 batters, 210 left, 327 right, 67 switch. STATE said
pending; the data says otherwise, and the note is corrected. Point-in-time holds
as long as the split is fetched before first pitch, since a season split queried
this afternoon cannot contain tonight — but a historical backtest would use
season-final numbers and leak, so clean measurement is forward-accruing.
THE SPINE — regressed = (PA x observed + K x prior) / (PA + K), with K = 600 PA
and the prior being the hitter's OWN blended rate rather than the league's. The
question a platoon adjustment answers is whether he is DIFFERENT against this
hand than he normally is, so his own line is the correct null and a hitter with
no evidence of a split correctly gets nothing. K is deliberately conservative:
platoon skill is famously slow to stabilise, with the half-signal point for
right-handed batters near a thousand PA.
THE MAKE-OR-BREAK TEST, both halves. A .310 average against left-handed pitching
on 30 PA gets 4.8% weight and moves the projection by 0.003 — essentially
nothing, which is the correct answer rather than a limitation. The SAME .310 on
400 PA gets 40% weight and moves it by 0.023, eight times as far. A test asserts
that ratio stays above five, so if the regression ever breaks the suite says so
instead of the projections quietly drifting onto noise.
Real data behaves exactly as the mechanism predicts and is worth recording:
Josh Bell hits .259 against lefties and .248 against righties, which looks like
a platoon split until the sample speaks — 126 PA earns 17% weight and the
adjustment lands at 1.005. Aaron Judge, 76 PA against lefties, comes out at
0.999. Neither is material. Most hitters will get nothing from this adjuster,
and that is the honest output, not a failure.
Honest-absent has five distinct routes, all returning exactly 1.0: no batter
handedness, no pitcher handedness, no splits, a stat platoon says nothing about,
and a missing side falling back to the prior rather than to zero.
INDEPENDENT of the environment. Park and weather compose into one coefficient
because they both describe the stadium; platoon describes this hitter against
this pitcher's hand, so it rides its own slot with its own label. Entangling
them would make both harder to attribute when the instrument scores them.
Directional, mirrored on the under, capped at 15%, and inverted for strikeouts
where a higher rate means a higher prop rather than a better hitter.
Tests 3729 passed / 300 suites, web build exit 0.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCNgGSt5qvcLxaeQqa7Zpj
builtbykev
6dd6f59481
