How a projection is made.
One engine prices every market: strikeouts, hits, total bases, home runs, game totals and the moneyline. It works one plate appearance at a time, then adds up the game.
1. Every hitter and pitcher, as rates
For each hitter and each pitcher the engine estimates how often a plate appearance ends each of eight ways: strikeout, walk, out, single, double, triple, home run, or reaching on an error.
- Expected outcomes count three times as much as actual ones. A ball's exit velocity and launch angle say more about the next month than whether it happened to fall in.
- Small samples are pulled toward a starting point. That starting point is the player's previous season where he has one, and the league average where he doesn't. Strikeout rates are trusted quickly (halfway after about 60 plate appearances); singles and triples take hundreds.
- The window is the last 90 days, each day counted the same. Recent games get no extra weight, because in our tests they didn't predict better.
2. The matchup, including handedness
Hitter and pitcher rates are combined with an odds-ratio formula, then adjusted for the platoon matchup. The adjustment is the league-wide gap for that pairing of hands (left-handed hitters, right-handed hitters and switch hitters, against each throwing hand), not each player's own split. A player's own split is mostly noise until thousands of plate appearances, and using it made predictions worse. Bullpens are priced as a mix of left- and right-handed relievers, weighted by how much each pen actually uses its lefties.
3. The fitted correction
A gradient-boosted model starts from those odds and learns a correction from 210 pre-game features: each hitter's and pitcher's results and swing decisions, the pitcher's velocity, movement, release and our pitch-quality scores, how the hitter has done against pitches shaped like tonight's starter's, and the pitcher's role. Trained on 2022 onward and tested walk-forward through 2026, it improved every market it touches in both test windows.
4. Park and pitch mix
Park effects come from real geometry (outfield distances and fence heights) and the air: density from observed temperature, dew point and pressure, plus wind. The park moves home runs fully, doubles and triples about half as much, and singles about a quarter as much. A batted-ball layer then turns each hitter's contact, built from balls in play only, into singles, doubles, triples and outs for tonight's park.
5. The game
Run totals and win probabilities come from an exact walk through every base and out state, inning by inning, rather than a random simulation. Player props use closed-form distributions over the plate appearances each player is expected to get. A starter's expected length comes from his starts only, never his relief outings.
6. Calibration
Where a market has earned one, a calibration curve maps the engine's number to the rate that actually happened on earlier dates. A curve ships only if it beats the engine's own number on later dates it never saw, and every curve is thrown away when the engine version changes.
The record
Every prediction is written before first pitch and frozen. Later builds can add the result and the closing price, never change the prediction. Every market carries a status computed from that evidence; see the model cards and the record.
What makes a play
Most projections are never plays. One qualifies only when all of these hold:
- The edge over the book clears the larger of 3 points and the market's own measured calibration error.
- The market is Validated or Promising. A Promising market needs twice the edge, and can never be a top-tier play.
- The price exists, is under two hours old, and the lineup is confirmed.
- For strikeouts, the model must also beat a coin flip at the book's own line on the live record. It doesn't yet, so strikeout plays are paused.
No stake is ever suggested.
What the engine does not use
- Weather in run totals. Weather shapes park air for batted balls, but the separate run adjustment has never been switched on; it needs its own held-out test first.
- Hot and cold streaks. Shown for context; not weighted.
- Team defense and bullpen fatigue. Both tested; neither improved totals. The study.
- Umpires. Too small to measure on the games we replayed.