Key Definitions
- USS
- Umpire Stolen Strike — A pitch outside the true zone that was called a strike, where the challenge EV is negative (offense wouldn't challenge). This is framing value that "survives" ABS.
- ULS
- Umpire Lost Strike — A pitch inside the true zone that was called a ball, where the challenge EV is negative (defense wouldn't challenge). This is value the defense loses to umpire error.
- Net ABS
- ULS Runs − USS Runs. Positive = benefits from ABS (recovers more than loses). Negative = hurt by ABS (elite framers whose stolen strikes exceed their lost strikes).
- Contested
- Pitches where challenging has positive expected value. Under ABS, these would likely be overturned — they don't contribute to USS/ULS.
Shadow Zone Definition
Like Statcast, we use each batter's personalized strike zone (sz_top/sz_bot) for the vertical boundaries, and the fixed 17″ plate width for horizontal. But our shadow zone differs from Statcast's zone numbering system in two key ways:
1. Ball radius adjustment (ABS true zone). Statcast measures the center of the ball (plate_x/plate_z). Under ABS, a pitch is a strike if any part of the ball touches the zone. To model this, we expand the zone edges by ball radius (~1.5″): horizontal at ±(8.5″ + 1.5″) = ±10″ from center, vertical at sz_top + 1.5″ and sz_bot − 1.5″. This is how ABS determines whether a challenged pitch would be overturned or confirmed. Statcast's zone numbering (1-14) does not include this ball-radius expansion.
2. Distance-based shadow band. The shadow zone is all pitches within 3 inches (0.25 ft) of those true zone edges — from either side. This captures pitches just outside the zone (where catchers frame for stolen strikes) and pitches just inside (where umpires sometimes miss calls). Statcast's shadow zones (11-14, 16-19) use grid regions that extend further out and don't distinguish inside-edge vs outside-edge pitches.
The zone is split into four edges (Inside, Outside, Top, Bottom) based on which true zone edge the pitch is nearest to. For the 8-zone grid view, we classify shadow pitches into zones 11-19 by position relative to zone corners and sides, matching the Statcast numbering convention.
Note on strike zone data: All years (2021-2025) use Statcast's operator-measured sz_top/sz_bot, set per-pitch when the ball is halfway to the plate. Hawk-Eye tracked strike zone measurements are expected in 2026 Statcast data. MLB's ABS challenge system uses separate formula-based boundaries (53.5% and 27% of batter height) not yet in public tracking data. Our USS/ULS model uses the Statcast operator values as the best available approximation of where ABS challenges would apply.
Umpire Stolen Strikes (USS)
USS measures strikes called on pitches that were actually outside the true zone — value the offense loses. Not all stolen strikes are equal — we weight them by game state using RE288 (run expectancy by count and base-out state).
A stolen strike on 3-2 with bases loaded is worth far more than one on 0-0 with nobody on.
Umpire Lost Strikes (ULS)
ULS measures balls called on pitches that were actually inside the true zone — value the defense can recover by challenging. This is the mirror of USS.
Under ABS, the defense can challenge these missed calls and recover the strike. ULS represents the recovery potential.
Net ABS Impact
The true ABS impact combines both sides:
- Positive Net ABS → Benefits from ABS (recovers more lost strikes than loses stolen strikes)
- Negative Net ABS → Hurt by ABS (loses more stolen strikes than recovers)
League-wide, the defense benefits: pitchers are getting squeezed (balls called on strikes) more often than they're stealing strikes. Only elite framers have negative Net ABS.
- Target low-confidence locations — pitches 1.5-2.5" off the zone where Hawkeye confidence is lowest
- Exploit early counts — 0-0, 0-1, 1-0 counts where challenge EV is often negative for the offense
- Sequence into steals — use pitch sequencing (e.g., breaking ball → fastball) to set up edge perception
- Avoid high-leverage traps — don't waste framing effort on 3-2 counts where every borderline call will be challenged
The catchers with negative Net ABS (Kirk, Bailey, Hedges) aren't "bad" — they're elite framers whose skill becomes contestable. Under ABS, the goal shifts from maximizing stolen strikes to maximizing uncontested stolen strikes.
Contested vs. Uncontested
Under the ABS challenge system, teams have limited challenges. We use Tango's challenge threshold to classify pitches:
- Uncontested (USS/ULS) — Pitches where the expected value of challenging is negative. These "survive" ABS because no rational team would spend a challenge on them.
- Contested — Pitches where challenging has positive EV. Under ABS, these would likely be overturned.
The threshold varies by count and game state. A borderline pitch on 3-2 is always worth challenging; the same location on 0-1 often isn't.
Sequence Effects
Pitch sequencing affects umpire perception. We categorize pitches into three buckets:
- FB — Fastballs (4-seam, 2-seam, sinker, cutter)
- BRK — Breaking balls (curveball, slider, sweeper)
- OS — Offspeed (changeup, splitter)
A fastball following a breaking ball gets called differently than a fastball following another fastball. This data quantifies that effect.
Platoon Edges
Inside/outside edge interpretation varies by batter-pitcher handedness matchup. The "inside" edge to a RHB facing a RHP is the pitcher's arm side — different visual geometry than LHP vs RHB.
Data Source & Qualifications
Built from Statcast pitch-level data. Pitcher qualification: 50+ shadow zone called decisions. Catcher qualification: 100+ shadow zone called decisions. Battery pairs require 30+ decisions together.
Methodology draws on work by Tom Tango (challenge EV framework), Baseball Savant (zone probability models), and RE288 run expectancy tables.