Do Full-Time At-Bats Improve a Hitter’s Performance?

Today, I’m going to take a stab to answer the simple question:

“Do hitters perform better with regular at-bats?”

While the question is simple, finding an acceptable answer is not so easy. While my final approach is far from “perfect”, it verifies the statement is true to a small extent.

The concept behind the rule-of-thumb is that with regular at-bats, a hitter can get into a groove and perform better. Usually, the idea references a hitter in a platoon.

Besides being in a platoon, he could be in the National League and used primarily as a pinch hitter. Anyone going from mainly being a pinch hitter to a full-time role will no longer occur the pinch hitter penalty.

To find a group of hitters to compare their stats, I took hitters from 2010 to present day jwho:

  1. Had between 10 and 50 PA in month #1 that prorates out to 60 PA to 300 PA for a season.
  2. They spent no time on the IL or moved to and from the minors in the first month. I wanted the hitter on an MLB club but not playing.
  3. Accumulated 90 PA (540 PA for a season) or more in the second month.

In all, 192 hitters made the cut. It far from ideal, for the main reason the samples have to perfectly fit into a month’s timeframe.

Average & Median Stat Change When Going From Part-Time to Full-Time At-Bats
Stat Average Median
K% -0.1% 0.7%
BB% -0.4% -0.2%
BABIP .009 .010
ISO .004 .004
OPS .013 .021

A 20 jump in OPS isn’t going to turn a hitter’s season around. It helps for sure, but hitting full-time is not a game-changer.

Again, the measurable differences can likely be explained away by the effects of pinch-hitting. All the gains (BABIP and ISO) are from making better contact that could be from seeing a starter more than just once.

To look at it another way, 102 (53%) hitters improved and 90 (47%) got worse with full-time at-bats. It’s really close to being 50/50.

On average, going full-time will give a hitter a small boost in their production, but not enough to turn around a hitter. From now on, I’m going to assume a hitter with a full-time role will just keep performing as they always have. Any expected improvement is negligible.

Jeff, one of the authors of the fantasy baseball guide,The Process, writes for RotoGraphs, The Hardball Times, Rotowire, Baseball America, and BaseballHQ. He has been nominated for two SABR Analytics Research Award for Contemporary Analysis and won it in 2013 in tandem with Bill Petti. He has won three FSWA Awards including on for his MASH series. In his first two seasons in Tout Wars, he's won the H2H league and mixed auction league. Follow him on Twitter @jeffwzimmerman.

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Jeff, I love the question and the approach to answering it. How can we control for selection bias? Most of these 192 “earned” the right to play full time, were underperforming their peripherals and someone saw all the hard-hit outs, or both. Could we do this with xWOBA instead as far as 2010?


Good point. I wonder if players who were thrust into more AB’s because of injuries to superior players offsets this, and to what degree. I guess I’m not sure I would assume most earned it (then again, Roberts choosing to sit Muncy for Hernandez vs LH for basically a year is more memorable to me than him finally giving Muncy the full time job). Either way, a good question leads to more questions, and that’s exactly what Jeff did here.

I’d be interested in seeing actual WOBA as well as the x stat, just because neither is perfect but both are insightful in their own way.