clock menu more-arrow no yes mobile

Filed under:

Some xwOBA Underperformance Goes Beyond Bad Luck

A deep dive into xwOBA, batted ball luck, and perceived over/under performance.

San Francisco Giants v St. Louis Cardinals Photo by Dilip Vishwanat/Getty Images

It’s been a rough year. There’s no denying it. The St. Louis Cardinals sit in last place in NL Central with the worst record in the National League. Yet there does appear to be some bad luck involved.

As a team, the Cardinals have a .324 wOBA, which ranks 12th in the majors, and a .338 xwOBA, which ranks 5th in the majors. That’s quite the gap. In fact, it’s the 6th highest gap of any team in Major League Baseball. That’s textbook bad luck, right?

In case you don’t know, wOBA, which stands for weighted on base average, measures a hitter’s overall offensive value by weighting each individual event according to the value it brings. For instance, a home run is more valuable than a single, although batting average and OBP treat them the same, and wOBA weights the two events differently than slugging percentage.

Basically, wOBA gives one single number to measure a player’s overall offensive contribution. And to make things simpler, if you know what a good OBP is, you know what a good wOBA is because they are on the same scale.

xwOBA, or expected weighted on base average, tries to determine what a hitter’s wOBA should be based on the exit velocity, launch angle, and sprint speed (only on balls that are considered topped or weekly hit) of each batted ball event. Essentially, the goal of xwOBA is to strip out the role played by defense to isolate how well a hitter is hitting the ball.

Why is this valuable? Well, because sometimes a hitter crushes a ball in the gap but Harrison Bader robs him of a double. While that well hit ball wouldn’t be reflected in his wOBA, it would be seen favorably according to xwOBA. Or, sometimes a left fielder climbs the wall only to watch the ball land 20 feet in front of him (I’m not naming names here). In that instance, the hitter’s wOBA sees a boost from the double but his xwOBA doesn’t since the ball wasn’t hit that well.

As a statistic, xwOBA helps filter out some of the noise that is, for the most part, out of the hitter’s control. So, subtracting a hitter’s (or a team’s) xwOBA from his wOBA is a great way to see if he’s been unlucky.

So, back to the question I posed earlier. The Cardinals have been unlucky, right? With the 5th ranked xwOBA in the game, they should surely be hitting the ball better, right?

My answer is yes. They have probably been unlucky. But it’s not as straightforward as you might think.

xwOBA is not a perfect stat. In theory it’s great and in practice it’s incredibly helpful, but there are still some holes in the stat to be aware of. I am going to discuss those and then move on to applications for the Cardinal lineup.

Hole #1 - Straightaway Fly Balls

Fly balls hit to the middle of the park have a tendency to fare much better in xwOBA than they do in wOBA. Across, the league straightaway fly balls have a .520 xwOBA this year, which is fantastic, but those same fly balls only have a .312 wOBA, which is slightly below the .318 league average wOBA.

I suspect the gap here is due to one simple fact - the walls are deeper in center field. xwOBA doesn’t factor in fence distance, so while a fly ball might clear the fence in left or right field, it could easily come up short when it’s hit to the deepest part of the park.

It’s important to be aware of this when trying to compare a hitter’s wOBA and xwOBA to determine if he has been unlucky. If a hitter tends to hit a well above league average amount of straightaway fly balls, then he’s likely to underperform his xwOBA by a decent margin. That’s not bad luck. That’s simply sub-optimal batted ball direction.

Hole #2 - Pulled Fly Balls

While xwOBA comes in much higher than wOBA on straightaway fly balls, xwOBA drastically undervalues pulled fly balls. The league wOBA on pulled fly balls this year is .866 which is nearly 200 points higher than the league xwOBA on pulled fly balls (.672).

The moral of the story here - if a hitter pulls a large chunk of his fly ball and is overperforming his xwOBA, you can’t just chalk it up to good luck.

This hole is a little more interesting to me. The underperformance on straightway fly balls has a simple explanation - fence distance. But that doesn’t work here. I will theorize that the average left field fence distance in major league baseball is about the same as the average right field fence distance. Even if that’s not true, it’s fair enough to say that centerfield is significantly deeper than both left field and right field.

So, if the answer here was fence distance again, you would expect to see the same xwOBA overperformance on opposite field fly balls, but that’s not the case.

The league wOBA (.243) on opposite field fly balls and the league xwOBA (.256) on opposite field fly balls are only separated by 13 points. I think we can rule out the fence distance hypothesis here.

If I were to guess, I would say that this hole has to do with batted ball spin. I don’t know the physics behind hitting a baseball to different parts of the field, but I would reason that hitting the ball out in front could potentially cause the ball to be hit with more backspin, thus allowing it to fly further in the air.

Balls that are hit to the opposite field tend to slice more, which means they have more side spin and thus may not be prone to the same xwOBA overperformance as pulled fly balls.

This is simply a theory with no numbers or hard evidence behind it, though, so I could be way off. But, regardless, if a hitter pulls a lot of his fly balls and is overperforming his xwOBA, he’s not due for regression.

Hole #3 - Opposite Field Ground Balls

Hitting a high percentage of ground balls to the opposite field is also a great way to outperform your xwOBA. Opposite field ground balls have a league-wide xwOBA of just .221, which is well below average, but the league-wide wOBA is actually well above average at .371.

There is a simple explanation for this one too. It’s the shift. Er...sorry...what shift? There’s no more shifting this year, right?

Well, if you’ve been watching any games this year, shifting hasn’t been eliminated, it’s just been toned down. Teams may be limited to 2 infielders on either side of second base but they are still shifting infielders towards the pull side. We haven’t really seen teams return to traditional defensive positioning, which is why the league average wOBA on ground balls has only dipped a bit from last year and not returned to pre-shift levels.

(Note: Mind the scale of the graph. The vertical axis doesn’t start at 0.)

When teams shift towards the pull side, they open up gaps on the opposite side of the infield. Thus, it makes sense that hitters are vastly outperforming their xwOBA on opposite field ground balls.

Hole #4 - Speed Matters

xwOBA is focused on how hard a hitter hits the ball and the angle at which the ball leaves the bat, and while those things are hugely important, speed makes a difference too. Speed can be the difference between an out or a hit on a weakly hit grounder. It can be the difference between a double and a triple on a ball in the gap. It can be the difference between a single and a double on a ball that’s somewhere not quite in the gap but not quite at the fielder either.

Players who are slow can often underperform their xwOBA for that very reason and players who are fast can overperform it by stealing hits and extra bases where a slower-footed player wouldn’t.

An update to xwOBA made a few years ago added speed as a consideration on balls that are considered topped or weakly hit, and, as a result, speed doesn’t have as strong of a correlation with xwOBA overperformance and underperformance anymore. Still, though, speed is not a consideration on other balls and fast runners can still steal extra bases on balls hit to the outfield.

Speed matters when considering wOBA vs. xwOBA performace, just not as much as it used to.

What This Means For the Cardinals

How much of a role has bad luck played in the slumping offense? I want to take a look at a few players with big gaps between their wOBA and xwOBA to show that not every underperforming (or overperforming) player can blame batted ball luck.

Willson Contreras

Among all players who have seen at least 200 pitches, the catcher has the 18th biggest underperformance between his wOBA (.284) and his xwOBA (.341). This year, that gap is the difference between hitting like Justin Turner, Max Muncy, or Anthony Rizzo, and hitting like...well...Willson Contreras.

There’s an abnormally large gap between Contreras’ wOBA and xwOBA so luck may still be playing at least somewhat of a role but there are other forces at play here. To be more specific, Contreras doesn’t pull any of his fly balls.

In fact, he has pulled just 10% of his fly balls this year, which is well below the league average of 24.5% and also well below his career rate, which usually hovers in the low-to-mid 20%.

He’s mostly made up the difference with opposite field fly balls and not straightaway fly balls, but that still has a huge impact on keeping his wOBA below his xwOBA.

Pulling fly balls may give a big boost to wOBA relative to xwOBA but not hitting pulled fly balls works in the other way too. If that’s confusing, think about it this way.

The league average wOBA on fly balls this year is .434 and the league average xwOBA on fly balls is .464. Essentially, the wOBA and xwOBA on opposite field fly balls is the same while wOBA is much greater on pulled fly balls and xwOBA is much greater on straightaway fly balls.

So, when a hitter sees his fly ball pull rate drop, he’s seeing fewer batted balls that balance out the wOBA gap while keeping all other types of fly balls the same. That equates to a lower wOBA.

So we can say that there’s been some bad luck involved but some of Contreras’s underperformance is due to the direction in which he hits his fly balls. If he can get back to pulling his fly balls at his normal career rate, I would expect to see his numbers start to bounce back.

Tyler O’Neill

O’Neill has been hurt for much of the season and just got moved to the 60-day IL so this section isn’t as relevant to the team as currently constructed. Still, O’Neill is worth some analysis because he’s doing the exact opposite of what a hitter should be trying to do.

Namely, he’s pulled all of his grounders and none of his fly balls.

O’Neill has hit 57.1% of his fly balls up the middle while hitting just 4.8% of his fly balls to the pull side and just 4.3% of his ground balls to the opposite field. Those numbers are so drastic that I’m not sure there’s any poor luck involved. That’s a surefire way to underperform your xwOBA by a lot. And that’s exactly what O’Neill is doing.

I want to point out that O’Neill has yet to reach 100 plate appearances this season so we’re still dealing with a small sample size. That could still normalize when O’Neill gets back on the field, but the point is that there are legitimate reasons for why he has underperformed his xwOBA this year.

Paul Goldschmidt

Unless you frequently check stats, it may be a bit surprising for you to learn that Paul Goldschmidt, the best hitter on the team is actually underperforming his xwOBA by 14 points (.376 wOBA vs .390 xwOBA). And that his xwOBA this year is actually much higher than his xwOBA last year (.367).

The interesting thing about this underperformance is Goldschmidt does everything right, at least in terms of doing the things that should keep him from underperforming his xwOBA. He pulls the ball in the air at an above average rate, he hits a below average percentage of his fly balls up the middle, and he hits an above average percentage of his grounders to the opposite field.

Paul Goldschmidt Luck Indicators

Player vs. Avg FB Pull% FB Center% GB Oppo% Sprint Speed (percentile)
Player vs. Avg FB Pull% FB Center% GB Oppo% Sprint Speed (percentile)
Paul Goldschmidt 28.6% 31.2% 19.7% 34th
League Avg 24.5% 37.1% 11.5% 50th

He’s a bit slow but not concerningly slow and we already know that sprint speed has been somewhat adjusted for. So, what’s the problem? Why is he underperforming?

It’s hard to say. Maybe he’s doing penance for overperforming his xwOBA by 52 points last year. Or maybe we can chalk it up to bad luck in this instance. And if it truly is bad luck, then it’s worse than it looks because everything in the batted ball profile says that Goldschmidt is a candidate to overperform his xwOBA, not underperform.

I’m never completely content pointing to luck as an answer but I’ll be watching to see if there’s any positive regression on the way for the Cardinal first baseman.

Nolan Arenado

Now that we’ve looked at 3 underperformers, I want to take a look at an overperformer. And Nolan Arenado is a serial xwOBA overperformer.

Nolan Arenado’s Career “Overperformance”

Year wOBA xwOBA
Year wOBA xwOBA
2015 0.371 0.336
2016 0.386 0.347
2017 0.395 0.366
2018 0.391 0.353
2019 0.392 0.343
2020 0.308 0.288
2021 0.336 0.311
2022 0.381 0.339
2023 0.348 0.320

How is that possible? Is he just a hack? Is xwOBA a bad stat?

The answer to the last two questions is a hard “no” but there is an explanation. Nolan Arenado likes to pull the ball. A lot.

The third baseman has pulled 38.6% of his fly balls this year and that’s not even an abnormally high rate for him. Arenado’s career high fly ball pull percentage is 41.8%, which he set back in 2021.

It’s no wonder Arenado has been a career long overperformer. Batted ball direction matters, especially on fly balls, and he is a master of pulling the ball in the air. That translates directly to his consistent xwOBA overperformance.

Final Thoughts

xwOBA is a great stat that can help provide a lot of context to a player’s success or his struggles but sometimes additional context is needed. xwOBA doesn’t factor in batted ball direction, and, as we have seen, that can create an xwOBA that can be a bit misleading in one direction or the other.

The Cardinals do truly have some players who have suffered from poor batted ball luck but they also have some players with xwOBAs that provide a bit of a misleading view into how well they really should be performing at the plate.

Thanks for reading, VEB.