Team-average defence failed the two-part gate for hits, and the reason was the
unit rather than the signal. A left-handed pull-ground hitter meets the first
baseman and the second baseman and almost nobody else, so a team total averages
in five fielders who will never touch his ball.
Both halves were already free on the host we pull from. Statcast publishes
spray x trajectory per hitter -- pull/straight/oppo crossed with ground/air,
608 hitters -- and the OAA feed already carries each fielder's position, so
per-position defence is a regrouping of data ingested last week rather than a
new source. Zero new sourcing, as the order expected.
Handedness is what joins them and getting it backwards would be invisible: pull
for a right-handed hitter is the left side, pull for a left-handed hitter is the
right side, so a model ignoring bats would send half the league's grounders to
the wrong infielders and still look like it was reading defence. A switch hitter
bats opposite the pitcher, which this does not resolve, so he is unreadable
rather than guessed.
Two properties the crude version could not express, both locked by test: two
teams with the SAME total defence read differently for a pull hitter, and a
ground-ball hitter and an air hitter read the same team in opposite directions.
Unmeasured zones are renormalised away rather than contributing a zero, which
would assert an exactly-average fielder standing there, and states
honestly what share of a hitter's contact we could actually read. Nothing
readable at all returns null, so the caller falls back to the base rate instead
of to an invented 1.0 that looks measured.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W1sivYNqY2TS5ftykmHBU9
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