Inner Half

With the introduction of Statcast technology in 2015, MLB evaluators have been able to unlock a new level of analysis. Understanding exit velocity, launch angle, spin rate or sprint speed allows students of the game to realize where certain players outshine others. If you watch enough baseball, you quickly learn there is much luck involved once a pitch is put in play. A sharply-hit line drive out will show up just the same as a chopped groundball to second base in the box score, but every onlooker will know which player put a better swing on the ball. Enter expected stats, metrics that allow us to get a gauge for what a player should have recorded.

There are a variety of expected stats, but I’m going to focus on the most basic and easiest to understand: xBA, xSLG and xwOBA for hitters, and xERA and xwOBA against for pitchers.

xBA, or Expected Batting Average, is a measure of the likelihood that a batted ball will become a hit, based on the batted ball’s exit velocity (EV) and launch angle (LA) and, sometimes, the batter’s sprint speed. Each batted ball is assigned an xBA based on all the comparable batted balls that have become hits since the implementation of Statcast in 2015. 

For example, let’s say a batter hits a line drive at 95 mph EV and 5 degrees LA. Because a batted ball of that combination of EV and LA has likely become a hit so many times before, it will be assigned a higher xBA. If you think about it from a high level, who doesn’t expect a ripped line drive to become a hit? Unless there is a superb defensive play, likely no one. 

With xBA, we can get a better idea of who is making quality contact or hitting the ball hard. Because the hitter has no control over what happens once the ball is put in play, their standard batting average could take a hit if the ball is caught, but xBA will reward the hitter for hitting the ball well.

If a hitter’s xBA > BA, the hitter is making better contact than his traditional average would suggest. If a hitter’s xBA < BA, the hitter may be benefitting from some subpar defense or lucky bounces.

On the flip side, xBA can be measured against pitchers. If a pitcher’s xBA against > BA against, the pitcher is overperforming his quality of pitching and his defense/luck could be helping him out. If a pitcher’s xBA against < BA against, he may be on the unlucky side or dealing with poor team defense behind him.

xSLG, or Expected Slugging Percentage, measures the likelihood of a certain result given the batted ball metrics, similar to xBA. In this case, every batted ball is given a likelihood to result in a single, double, triple and home run, based on comparable batted balls since 2015. 

Like traditional SLG, all hit types (1B, 2B, 3B and HR) are valued in the same fashion: a 2B is worth 2x as much as a 1B, 3B is worth 3x as much as a 1B, and a HR is worth 4x as much as a 1B and 2x as much as a 2B. 

Like xBA, xSLG should result in a more indicative measure of how a hitter is performing. If a hitter is putting good swings on the ball and making hard contact, their xSLG will be greater. With xSLG, we can get a feel for whether a hitter is overperforming or underperforming their standard SLG.

Pitchers also have an xSLG against, which is measured in the same way as it is for hitters. Ideally, a pitcher’s xSLG will be less than their actual SLG, meaning they are pitching better than a surface-level scrape would tell you.

xwOBA, or Expected Weighted On-Base Average, has the same function as the previously mentioned metrics. xwOBA uses the same hit values as standard wOBA, but is seen as more indicative than standard wOBA, as xwOBA removes defense from the equation and relies on probabilities. 

You probably have the gist of it by now, but if a hitter’s xwOBA is greater than their wOBA, they are probably getting unlucky. On the flip side, if a hitter’s xwOBA is less than their wOBA, they are likely overperforming based on their actual results.

Yes, pitchers also have an xwOBA against, which you can probably assume the meaning of by now.

xERA, or Expected Earned Run Average, is a simple 1:1 translation of xwOBA, converted to the ERA scale. Don’t get it confused with xwOBA against, though. xERA is meant to read like ERA and give an idea of whether the pitcher is over or underperforming their ERA.

What xERA tells us is what the pitcher’s ERA should be based on the results a pitcher gets. It is all but guaranteed that a pitcher will run into bad luck or bad defensive plays behind him, but xERA attempts to put a number to the actual quality of their pitching, minus the defense and luck.

If a pitcher’s xERA > ERA, the pitcher is probably benefitting in some way by their team’s defense or luck. If a pitcher’s xERA < ERA, the pitcher is likely getting unlucky or has poor defense behind him.

Examples

Hitter Example from 2025: Juan Soto (per Baseball Savant)

BA: .263 –> xBA: .300

SLG: .525 –> xSLG: .628

wOBA: .390 –> xwOBA: .441

One of the premier hitters of the game, Soto had an up-and-down first season in Queens. What happens when we take a look at his expected stats?

First, his .300 xBA > his .263 standard BA, meaning his underlying contact quality was better than the stat sheet would make it seem. Second, his .628 xSLG > his .525 standard SLG, meaning he should have produced a bit more in terms of total bases, based on his quality of batted balls. Third, his .441 xwOBA > his .390 wOBA, meaning Soto should have been even more elite in terms of plate production than he was already.

So, even when a hitter has a “down year” like a lot of people thought for Soto, he might have just been getting unlucky and actually was putting good swings on the ball.

Pitcher Example for 2025: Dylan Cease

ERA: 4.55 –> xERA: 3.47

BA against: .239 –> xBA against: .222

SLG against: .399 –> xSLG against: .372

wOBA against: .313 –> xwOBA against: .297

Subject to some trade rumors at the deadline, Dylan Cease’s 2025 numbers weren’t the best he’s put up in his impressive young career, but the expected stats suggest he was better than they indicate.

His 3.47 xERA > his 4.55 standard ERA, so Cease probably ran into some bad luck or suffered from bad defense. His xBA against, xSLG against, and xwOBA against all > his standard BA against, SLG against and wOBA against. In short, Cease did a better job limiting hard contact and quality batted balls than his standard numbers say he did.

Conclusion

When evaluating a player’s season, it would be wise to take a peak at the player’s expected stats. What you can take away from a player’s expected stats is how well they were hitting the ball or, for pitchers, how well they limited well-hit balls. Sometimes, baseball comes down to forces that are out of the pitcher and hitter’s control, and sometimes it comes down to plain luck.

You can find expected stats on sites like FanGraphs or Baseball Savant.