Does A Starter’s Improved Walk Rate Stick?

Today’s mini-study is brought to you by Austin Gomber and Tarik Skubal. Both struggled to start the season with April ERA’s over 6.00, but I noticed both have righted the ship with May ERA’s near 3.00. Excessive walks caused their early struggles but both got them under control.

With the regression always looming, I wondered how much of the May gains should be given back in June. After looking through the data, not as much as I expected.

I have an unfounded belief (possible future article) that most of a pitcher’s changes and/or improvements happen early in the season. If a pitcher is productive, he won’t change much and keep using the same approach. If a starter is not performing, he’ll need to adapt or be sent to the minors, the bullpen, or just released. Pitchers worked over the offseason on new pitches or deliveries and results of these adjustments don’t show up until games start. For these reasons, I wanted to just focus on April to May improvements and if they held in June.

The data requirements are simple. The starter must have made three starts in each month. They had a walk rate over 4.5 BB/9 in April and under 3.0 in May since 2010. And here is how that turned out.

Note: I’ll use the median change from now on since a few high ERA could distort the average values.

On average, these pitchers experienced a halving of their walk rate and gave back just 0.5 BB/9. By not giving back much of the gains, many stayed below the 3.5 BB/9 acceptable threshold for rosterable starters. In all, 73% of the pitchers kept their walk rate under 3.5 BB/9 in June.

After finding the gains, I wasted a ton of time. I wanted to determine if the pitchers who kept their gains had any similar traits. I found nothing. Zero. Damn. For now, the level of regression to April numbers seems random.

So besides Skubal and Gomber, here are the starters who met the above walk rate improvements from April to May this year.

Pivetta is the one I’m keeping an eye on going forward. While his owners enjoyed good April numbers, they may be getting weary of his May 4.82 ERA. Besides the improved walk rate, his strikeouts are up (8.8 K/9 to 10.9 K/9). The combination has pushed his K%-BB% from 8% in April to 21% in May. The value was good for 24th in May among starters ahead of Trevor Bauer and Lucas Giolito. The walk gains have pushed Gomber up to #12 and Skubal up to #8 on the same list.

Overall, I’m extremely interested in any pitcher making significant early-season gains with their walk rate. Since the gains usually stick, the pitcher’s improvement might be masked by their early struggles.