Platoon Splits Explained: Why Handedness Matters

A right-handed pitcher's breaking ball moves away from a right-handed hitter and into a left-handed one, and the hitter sees the ball out of the hand later or earlier accordingly. The result is one of baseball's most reliable patterns: hitters are better against pitchers who throw with the opposite hand. Switch-hitters always take the favourable side.

How the model uses it

For each lineup, the model counts the share of plate appearances (weighted by batting-order slot) that come from hitters with the platoon advantage against the opposing starter. That share only matters for the part of the game the starter is expected to pitch; the bullpen will bring in its own matchups later. It also measures each team's hitting against left- and right-handed starters, relative to its own overall level and regressed hard toward zero, which is shown on each team page.

Why not use each hitter's own split?

Because it takes an enormous amount of data to measure. A regular faces left-handers in only about a quarter to a third of his plate appearances, so even a full career gives a noisy estimate of his personal split. Most of the difference between two hitters' splits in a given year is luck. Public research consistently finds that regressing individual splits heavily toward the league-wide platoon effect is more accurate than trusting them — so the model does exactly that at the lineup level instead of pretending to know each hitter's true split.