Guide / Definitions
Arsenal metrics
| Metric | Definition |
|---|---|
| Tunnel Spread | Measured in inches, this is the usage-weighted average distance between the pitcher's pitch types 35 feet in front of home plate (the chosen tunnel point). For every pair of pitch types, we take the straight-line distance between their average locations at 35 ft, weight it by how often that pair is thrown, and average across all pairs. A lower value means the pitch types appear in nearly the same location at the tunnel point. |
| Plate Spread | Uses the same usage-weighted pairwise distance as Tunnel Spread, but measured as the pitches cross home plate instead of at the tunnel point. Measured in inches. Higher means the pitch types finish farther apart by the time they reach the plate. |
| Late Divergence | Plate Spread minus Tunnel Spread — the extra separation the pitches gain between the 35-ft tunnel point and home plate. Measured in inches. A high value indicates that the pitches look alike early and then split apart late. |
| Velo Spread | The usage-weighted average velocity gap between the pitcher's pitch types. For every pair of pitch types, we take the difference in their average velocity, weight it by how often that pair is thrown, and average across all pairs. Measured in mph. Higher means more speed separation across the arsenal. |
| Arsenal Entropy | The Shannon entropy of the pitcher's pitch-usage mix — how many pitch types he throws and how evenly he mixes them. Higher means more pitch types thrown in more even proportions; lower means the pitcher leans heavily on one or two pitches. |
A note on the 35-foot tunnel point. The tunneling metrics above are all measured 35 feet in front of home plate, rather than at the hitter's traditional “decision point” of roughly 25 feet — the spot where a batter is generally thought to commit to his swing. We measure at 35 feet because Late Divergence correlates more strongly with both Whiff% and Chase% there than it does at 25 feet. That may suggest that a pitcher's deception at 35 feet, while the pitches are still bunched together and before the hitter has fully committed, is actually more important than deception at the widely cited decision point.