While hiking a few months ago Jeff and I were talking about baseball and specifically, the fastball. To me, it feels like once you get into the upper 90’s the speeds seem to be idolized in a way that may not make sense. Does the actual velocity matter? Is it that much harder to bat against a ball going a mile per hour faster? In essence:

How much better is a 96mph fastball than a 95mph fastball?

I couldn’t find this anywhere so I took a stab at doing it myself. I looked at three stats:

Contact% = Pitches on which contact was made / Swings

= Pitches on which contact was made / Swings Swing% = Swings / Pitches

= Swings / Pitches SwStr% = Swings and misses / Total pitches

These stats were chosen because the made sense to me and could be calculated from pitches individually, as opposed to needing the data from the entire at bat.

For the data, I used data from Clayton Kershaw’s pitches over his entire MLB career (2008-present). Why Clayton Kershaw? Because I don’t know baseball very well and someone told me he throws both fast and a lot. This gave me ~16,000 data points — here is the results from that data:

Contact%:

Contact%, defined by [Pitches on which contact was made / Swings] goes down, as most people predicted. For every mph you increase the velocity the contact percentage drops an average of 2%, which seems significant. In essence, its harder to make contact with faster pitches.

Swing%:

Swing%, defined by [Swings / Pitches] goes up, as everyone I asked predicted. For every mph you increase the velocity, batters swing about 3% more often. Looking at the range it shows you that the fastest of his pitches are swung at about 40% more than his slowest pitches, which is a huge gap. Batters swing more at faster pitches.

SwStr%:

SwStr%, defined by [Swings and misses / Total pitches] also goes up, as everyone I asked predicted. This is the most striking to me — batters swing and miss at balls on average twice as much when the fastball is 95mph compared to when its 92mph.

Conclusion:

Faster fastballs are better. The data we looked at clearly showed that all three of these stats get better for the pitcher as you increase the velocity. And the titular question can be answered now:

A 96mph fastball is going to be made contact with 2% less often while being swung at 3% more often and swung on and missed about 1.5% more often.

Notes:

The code can be found here. I used python 3.x with pandas and matplotlib to parse it.

Data points are taken by putting the slowest 200 pitches into a bucket. That bucket is then checked for the stat in question and the their velocities are averaged. The bucket size of 200 was chosen because it gave was the smallest number that showed the results without being unnecessarily noisy.

Obvious extensions would be doing this for other pitchers (does it extend beyond 96? Is every pitcher’s graph similar?) and other stats (Z/O-Swing, Z/O-Contact, basically everything here)

Addendum:

I added results from Chris Sale and Felix Hernandez to get a bit more diversity. Below are their graphs: