With AAA Pitch Clock, Stolen Base Major League Adjustment

On this past weekend’s On The Wire Podcast, there was a discussion surrounding Esteury Ruiz stolen base numbers and the effect of the minor league pitch clock. The gist of the situation can be taken from this Tucker Davidson tweet thread.

Some Take Aways From the newly installed pitch clock. I’ve made 3 starts with it and each time it’s been a different experience. Overall not a fan. Let’s talk about the good and the bad. (Thread) @jjcoop36 @JeffPassan

— Tucker Davidson (@tucktuck6) April 29, 2022

The rate of stolen bases is up in the minors as soon as they implemented the change. With the numbers up, I wanted to calculate a simple rule to see how much I should expect a hitter’s stolen base rate to change once promoted, especially rabbits like Ruiz.

For the calculations, needed to set some ground rules. I limited my sample to this season’s hitters who stole four or more bases in AAA. Additionally, they had to go on and have 30 MLB plate appearances. In the end, 40 players met the requirements. I know the sample isn’t the biggest, but I’m just looking at how a hitter’s stolen base rates should change.

For the first test, I compared the stolen base attempt rate ((SB+CS)/(1B+BB+HBP)) in AAA and the majors. In AAA, the median stolen base rate among these guys was 19.5% and it dropped to 10.8% in the majors. When comparing the ratio change (MLB rate/AAA rate) it worked out to 53.5%. So a hitter’s stolen base rate should drop about 50%.

The next test was to look at the success rate. In AAA, this group had an 80.2% success rate and it dropped to 72.1% in the majors. A success of 80% would help a team while 72% hurts their chances of winning.

One final test, what was stolen base per plate appearance change. This value is a quick rule of thumb that takes into account attempts, success, and changes in on-base rate. This value works that the MLB rate is 43% of the AAA numbers.

So going back to Ruiz, here are his expected stolen base rates knowing what he did in AAA.

Here are the stolen base per 600 PA values for our projection systems.

His projections are close but just a little lower than the value I found. Maybe I should just follow the projections.