The Normalization of Z-O: Do Discipline Gains Hold?

Discipline at the plate is often a huge driver for success over the course of a season. From a fantasy perspective, it can be difficult to utilize outside of just an overall gauge for a hitter’s success.

I saw an interesting exchange on plate discipline data relating to zone and outside zone swing percentages as an indicator for a potential breakout. A post highlighting patience as the potential reason behind Pete Crow-Armstrong’s MVP-caliber season noted his O-Swing% is down to a career low 34%.

Eno Sarris suggested this number should actually revert closer to 41% by the end of the season.

“I’d bet on regression there. Z-O is amazingly sticky. I found that 62/70 qualified that saw their z-o decline in the first half went back to their norms in the second half. You can also see his Z-O improvement is smaller than just his O swing improvement. Pretty sure this is inherent.”

This piqued my interest, and I would like to build a chart that shows how hitters are performing relative to their baseline from the previous two seasons. How inherent is an approach? Are there hitters breaking away this year? Can we use this to indicate regression or success?

What is Z-O?

A quick run-through of what we mean by Z-O for those who may be unfamiliar. Z-O gap is the difference between a hitter’s Z-Swing% (how often they swing at pitches inside the strike zone) and their O-Swing% (how often they swing at pitches outside it). A higher number means a hitter is doing a better job of distinguishing balls from strikes with their swing decisions: attacking the zone and laying off pitches outside.

Take two hitters from 2025 as a reference point. Kyle Tucker posted an 18.6% O-Swing and a 68.6% Z-Swing, giving him a Z-O of 50.0, one of the best in baseball. He swung at fewer than one in five pitches outside the zone and nearly seven in ten inside it. Javier Baez posted a 41.6% O-Swing and a 65.5% Z-Swing, a Z-O of 23.9. His zone swing rate was only 3 points lower than Tucker’s, but he chased more than twice as often.

PCA, who had become somewhat notorious for his O-Swing% in particular, had a 41% chase rate, one of the highest among regulars, but his 80% zone swing partially offset it, leaving him with a Z-O of 39, a league-average gap.

The Chart

Every dot’s x-position is measured against that specific hitter’s own 2024-2025 Z-O average. Zero is the standard each hitter has set for themselves in previous seasons.

Cal Raleigh at -8.4 means his first-half 2026 discipline is 8.4 points worse than his own established level from 2024 and 2025. James Wood at +2.7 means he is 2.7 points above his own level from previous seasons. They’re not being compared to each other or to a league average; each hitter is compared to their own baseline from 2024-2025 data.

Sarris’s point is that most hitters by the end of the season should converge toward zero, due to the inherent approach of each hitter. You can hover on the dot to see the exact 2024-2025 baseline, 2026 Z-O, and the deviation. The y-axis is wRC+ to separate the dots corresponding to their performance as a hitter so far this year.

How much of a first-half discipline swing actually holds?

Using 2024 as the baseline, I measured every qualified hitter’s first-half 2025 Z-O deviation from their own established level, then checked how it compared in the second half. 225 hitters qualified with at least 150 baseline PA and 100 PA in each half.

Improvers, hitters whose first-half Z-O sat at least 2 points above their 2024 baseline, kept just under half the gain on average. The mean deviation went from +4.4 in the first half to +1.7 in the second. Decliners, those whose Z-O dropped at least 2 points below baseline, recovered about a third of their decline. The effect is regression to the mean, pulling both sides back toward the hitter’s established level, backing up the stickiness of Z-O as an approach stat. It is hard to get away from once it’s been established by the hitter at the big-league level.

Fantasy Relevance

So how can we use this for fantasy? When a hitter’s Z-O moved above their baseline in the first half, they kept just under half the gain by the second half. When it dropped below, they recovered about a third of the decline.

On the far ends of the spectrum, this emphasizes the struggles of hitters like Cal Raleigh (-8.4) and Bo Bichette (-8.9), whose plate discipline has degraded well beyond their established levels this season. But the 2025 base rate says about a third of that decline recovers, so expect Raleigh to claw back some of that Z-O in the second half without necessarily getting all the way back to his baseline. Players whose plate discipline numbers are down can still be productive, as Ben Rice, Gabriel Moreno, and Max Muncy have shown, and a partial discipline recovery could push them further.

On the other end, hitters sitting well above their own baseline face the opposite problem. Jake McCarthy at +7.6 above his established level should expect about half of that to disappear in the second half. Jordan Walker and James Wood, both having career years that have been underpinned by discipline gains, but the base rate suggests they are partially temporary.

But crucially, the takeaway is not to put too much into small samples of approach-based stats when looking for pickups, and vice versa when one of your hitters is in a slump. Eventually, over the course of the season, the Z-O will generally normalize. For fringe waiver wire pickups, it could be an opportunity to capitalize over a short period, but don’t expect it to hold.

This will definitely be one to come back to at the end of the season to see how many converged to zero and who established a new baseline for themselves in 2026. But it appears as if once you have your approach, it is very difficult to shake off.