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Benefit of the Doubt: How Pitch Speed and Movement Affect the Zone

As noted in previous episodes of Benefit of the Doubt, the strike zone as called is significantly modified by the behavior of the pitch and the context in which it is thrown. Today we'll look at the characteristics of pitch classification and its components: velocity and break.

At first glance, it would appear that off-speed pitches grow the zone. Take a look at the chart below, which compares the zone for off-speed pitches (in blue) to the zone for fastballs:


While the off-speed zone is clearly larger than the fastball zone, it is also higher. This is important, as only 25% of pitches thrown in the legal zone arrive in the top third. So while off-speed pitches are gaining more favor in the top third of the zone, they lose favor in the bottom third (which sees over 50% more pitches). Moreover, off-speed pitches left high in the zone are far more likely to make contact or elicit a swing, and thus have no impact on real-world zone advantage.

Below we see the zone advantage numbers for off-speed vs. fastball pitches, as well as pitch classification divided into three velocity categories: slow (under 85.7 mph), medium (85.7-90.8 mph) and fast (over 90.8 mph). 

Pitch Class Zone Advantage Speed Off-speed Fastball
Off-speed -1.47% Slow -1.38% 2.55%
Fastball 0.77% Medium -1.96% 1.27%
Fast -0.73% 0.22%


Note: Fastball class includes four-seamers, two-seamers, sinkers, cutters. Off-speed class includes change-ups, sliders, curveballs, knuckle curves, splitters, screwballs, and knuckleballs. Zone advantage favors the pitcher, so higher numbers mean a more pitcher-friendly zone.

The numbers show that off-speed pitches will yield ~1.5 additional balls and fastballs will yield ~0.8 additional strikes over the course of 100 called pitches. Digging deeper, we find that the slowest fastballs and the fastest off-speed pitches are the most likely to earn the benefit of the doubt, while medium-velocity off-speed pitches and fast fastballs are the least favored. The differences are rather stark: slow fastballs will earn the pitcher 4.5 extra free passes per 100 vs. medium-velocity off-speed deliveries.

Movement—in both absolute and regular terms—is also significant, albeit harder to discern. To demonstrate this effect, I split horizontal and directional movement values into three categories and calculated the following zone advantages:

Horizontal Deflection Zone Advantage Vertical Deflection Zone Advantage
Strong -0.19% Strong -0.72%
Medium 0.48% Medium 0.03%
Weak -0.47% Weak 0.51%
3B Side 0.30% Upward 0.62%
Straight 0.18% Flat 0.17%
1B Side -0.65% Downward -0.98%


And when we plot these variables against each other, we see some obvious trends:

Absolute Break Weak Horizontal Medium Horizontal Strong Horizontal Directional Break 3B Side Straight 1B Side
Strong Vertical 1.28% 0.84% -1.11% Upward -0.40% 1.41% 0.70%
Medium Vertical 0.15% 0.51% -0.41% Flat 0.75% 0.51% -1.02%
Weak Vertical -1.52% -0.22% -0.07% Downward 0.43% -1.60% -1.30%


For best results in the strike zone, throw pitches with weak horizontal break and strong upward break—not that the zone itself should be the pitcher's primary concern.

That's all for this week. As always, feel free to submit requests if you'd like me to investigate something specific. 

Previous episodes in the Benefit of the Doubt series

Data originate from Darrell Zimmerman's SQL-based PitchFX databaseStatistical models provided at the Rational Pastime site.