Strikeout and Walk Adjustments From Minor League Rules

This past season, the minor leagues experimented with several rules including pre-tacked baseballs and automatic strike zones. The following is a look at how those rules changed the amount of expected production from players.

First off, I’m not going to weave a narrative around this data dump. There is no polishing this turd. The information can be referenced later as fantasy managers begin to dive into 2023’s results while preparing for next season.

I examined the differences by comparing the same hitters over two different time frames. Additionally, I used some combination of the median, average, and weighted average.

Pre-tacked Baseballs in the Southern League (AA)

This rule change involved the league using a pre-tacked (sticky) ball from the season’s beginning to July 13th. This test only happened in the Southern League (AA).

First, here are the matched pitcher pair changes from 2022 to the first half of 2023. One note to remember is that, on average, these pitchers should improve by repeating the level, so their stats should normally improve.

While walks were up a bit with the new ball, the big mover was strikeouts which jumped 1.1 K/9. I have no pitch data from the league to know for fure, but most likely everyone’s pitches got more movement.

Now, here is the adjustment when the pre-tacked ball was removed.

The rates almost bounced back to previous levels.

Robo Umpires used in AAA (Includes Late Change)

In AAA this season, every game was called in some way by a pitch-tracking system. For the first half of the week, all balls and strikes would be called by the pitch-tracking equipment. In the week’s second half, the teams could challenge a certain number of pitches with the umpire calling the game as normal. Then on September 5th, a change was made to the system where the strikezone was adjusted based on the hitter’s height (I have zero idea why this wasn’t done previously).

For reference, here are the 2022 changes in pitching stats from AAA to MLB.

And here are the 2023 stats.

The big difference this season was that prospects making the jump from AAA to the majors saw their walk rate drop some. Historically, pitcher walk rates increased because of harder competition but this year’s results were the complete opposite.

Sticking with pitchers, here is how the rates compare from the static to the height-adjusted strike zone. Not much analysis can be done with this variable because of the lack of samples in the second dataset. For pitchers, ~600 innings of matched data was used, or the equivalent of four starters throwing 150 innings.

A small change can be seen across the board with pitchers taking advantage of the correct strikezone.

For the hitters, their plate discipline just fell apart even worse once they got the majors.

Both the walk and strikeout rates got worse at the same rate.

One final piece of information is needed, if the static zone hampered different pitcher heights. Here is how pitchers based on height (<70″, 70″ to 74″, >74″)

The results are mixed. Tall hitters got an advantage in AAA with a small strike zone and saw their strikeouts go up more than the others. The one I can’t explain is why short hitters saw a major drop in walk rate. Their strike zone should shrink going to the majors and thereby getting more walks. One explanation is that the data sample is too small. Another one is that in AAA, the short guys are used to walking since the pitchers there can’t find the strike zone but struggle once they get to the majors. I really don’t know.

Overall, I see two major keys to consider when doing offseason evaluations. First, know that pitchers struggled with walks in AAA and their walk rate should be lower when transitioning in the majors. Second, a hitter’s overall AAA plate discipline might have “improved”, but those gains will evaporate in the majors.