A while back I enjoyed reading a Fangraphs piece from Ben Clemens highlighting Yainer Diaz.

In the blog, Clemens presents optimism for Diaz’s offensive profile given his strong Run Value performance on pitches in the middle of the plate. A high hard-hit rate drives this performance, yet he is held back from a higher ceiling due to hitting so many of these heart pitches on the ground. Squandering so many of these opportunities that come in the middle of the plate, in combination with one of the higher chase rates in all of baseball, currently prevent Diaz back from unlocking true offensive success. Should he flip that switch, and start lifting the pitches he should (those over the heart of the plate), he’d become a different player.

I agree with Clemens, though. In situations like this, with clear low-hanging fruit and a path to significant improvement at the plate, us analysts can be overeager and “breeze over how difficult the change is”. So I decided to do a little extra digging.

Not only being interested in how likely a profile like Diaz’s might be to lower the GB%, but also whether doing so might discouragingly come with expected tradeoffs to what he is currently doing well: impact quality. Looking at hitters that decreased their GB% on pitches in the middle of the zone from one year to the next, we don’t see a strong correlation between degree of the GB% rate improvement and impact quality metrics like Hard-Hit% and SLGcon in the middle of the zone. This suggests that these metrics move on their own, rather than as a consequence of GB% improvement. I had thought that a tradeoff might exist, and I expected a stronger correlation between these metrics. But instead the numbers signal that any significant change in GB% would more likely be due to other circumstances, rather than as a direct tradeoff with impact quality. A positive sign for his development path!

However, we do see a moderately strong relationship between GB% and Sweet-Spot% (R = 0.23) on middle pitches. This makes sense intuitively. Given that a player starts hitting ground balls at a lower rate, it’s very likely we’d see a lot of those not only hit in the air, but also between the optimal launch angles. But nonetheless good to see it’s not all going towards whiffs or weakly hit pop-ups.

Going even further then to quantify the relationship between GB% and Sweet-Spot%, a simple linear regression tells us that for every 1% decrease in GB% in the middle of the zone, we would expect roughly a 0.469% corresponding improvement in Sweet-Spot% on those pitches. So for a 5% decrease in GB% we’d expect ~2.3% increase in Sweet-Spot%. That may not seem like a lot, but to add some perspective using a random baseball savant search, the difference last season between Juan Soto’s 28th percentile Sweet-Spot% and Cal Raleigh’s 60th percentile was just 2.4%.

Ok but how often are guys actually making a 5% improvement in GB% from one year to the next, where we might be optimistic for Yainer Diaz to do the same? In player seasons since 2021 (meeting a minimum of 60 BIP in the middle of the zone), 35% of hitters did so. It’s by no means a majority of players that are doing it, but I would still call that a sizable amount. This includes hitters of all profiles, so maybe drilling into a cohort more specific to a profile like Diaz’s and finding the rate of sustained change would be worth a future look. I found rates close to this when doing some research in a similar area a while back with a more specific subset of hitters, albeit from an overall GB% standpoint.