Slider Effectiveness & Spin: Unexpected Results

I heard in passing from a credible source:

“The effectiveness of a pitcher’s slider relies on it having the same spin as his fastball.”

I figured it would be an easy test and could help to immediately identify top-rated sliders. After looking at the data every conceivable way and came up with the following conclusion: publicly available spin information has near ZERO correlation to a slider’s effectiveness. But while rooting around, I did find two factors which do matter, fastball velocity and the difference in slider and fastball velocity.

The theory behind the quote is that a hitter has a tougher time differentiating a fastball and slider if they are spinning at the same rate. So, the closer the difference, a higher chance for a swing-and-miss.

I compared 2018 pitchers with at least 200 sliders and 200 four-seamers thrown. Then, I compared just the difference, the absolute value of the difference, square of difference. Nothing tangible. Nothing matched.

It just might be something I’m missing with an input. For example, a factor I don’t have available is the spin axis, so I don’t know how much the spin is working vertically and horizontally. This information is not publicly available but could be a major input.

Then I began to add in other factors to see if it was a combination of slider spin and pitch velocity. Zip.

Finally, I added in fastball velocity. First, slider velocity and swinging-strike-rate (SwStr%) had an r-squared of .014 and that was the highest correlation I could find before I began examining fastball velocity. The r-squared between fastball velocity and the slider SwStr% was .20. The faster a pitcher throws, their slider is more effective. With so many other factors in play (e.g. deception, spin axis, perceived velocity), I’m surprised I was able to get that high of a correlation.

I started incorporating fastball velocity with the slider metric. I quickly found that the difference between fastball and slider velocity to also be a “major” factor. The change wasn’t linear but instead, it was a parabolic change above and below a 10-mph difference. Simply, throwing as fast of fastball as possible with a slider 10-mph slower is are the keys to the slider’s success.

To simplify the parabolic equation, I used the following equation:

Absolute Value (Fastball Velo – Slider Velo – 10) = ABS Difference

When combining the two factors, the r-squared be .25 and this equation:

Slider SwStr% = -.00643 * ABS Difference + .00918 * Fastball Velo – 0.6778

Going over the projected results, the ABS difference component has a standard deviation of 1% and the Fastball Velo has a 2% standard deviation.

Overall, I was a little disappointed that spin rated didn’t matter. On the other hand, I found a couple of factors to help determine the effectiveness of slider, especially its effectiveness in relation to the fastball. There is a possibility I’m missing something and please let me know in the comments.