Yesterday I introduced a probabilistic model of the strike zone using PITCHf/x data. Today, I'd like to use that data to see how speed effects outcomes.

The model is built on three parameters, the X and Z coordinates of where the ball crosses the plate and the handedness of the batter. A positive probability outcome is calculated for each set of parameters, positive from the pitcher's point of view. So any pitch that does not result in a ball or a hit credited to the batter is a positive outcome.

For any set of pitches, the sum of the probabilities provides the expected number of positive outcomes. We can then compare that to the actual number of positive outcomes to see if the set of pitches was effective. Higher than expected positive outcomes indicate an effective set of pitches. Lower than expected positive outcomes indicate the opposite.

For this study, pitches are divided into sets by speed, rounded to the nearest mile per hour (MPH).

The following table shows the effectiveness of speed on pitches. The column definitions:

• Speed, in MPH. There must be 200 pitches at that speed to be included in the table.
• Pitches, total for that speed.
• Positives, the number of pitches with a positive result
• Pct. Pos., the percentage of the total pitches resulting in a positive outcome.
• Exp. Positives, the expected number of positives for those pitches based on the model.
• Exp. Pct., the percentage based on Exp. Positives.
• Ratio, 100*Positives/Exp. Positives. 100 meets expectations, over 100 exceeds expectations.

I did not expect this result. In general, I would have guessed that faster pitches would yield better results. This only appears to be true at very high speeds, however. Batters appears to find pitches thrown 97 MPH or better difficult to handle, but they have few problems with pitches in the low 90's. The most effective speeds for pitchers are between 75 and 85 MPH.

Note, however, that in terms of percentage, high speed pitches produce more good outcomes than low speed pitches. Low speed pitches result in a better than expected outcomes for a given location. Getting batters to swing at pitches in the dirt will do that.

This result seems to be in line with the idea that all major league batters can hit a fastball. How they hit the off-speed pitches, however, is what makes them a major league hitter. Twins fans should be very happy, given this data, that Francisco Liriano is bringing back his changeup.