Noteworthy xwOBA-wOBA Pitching Leaders

Sam Chinitz reviews the 2020 starting pitching leaders in xwOBA (expected weighted on base average) to decide which starters might see regression, both positive and negative in the coming season.

One of the oddities brought on by the shortened 2020 season was the number of qualified pitchers with huge gaps between their xwOBA and wOBA. In 2019, only seven qualified pitchers posted a wOBA at least 0.050 points away from their xwOBA in either direction, a number that ballooned to 52 pitchers in 2020.

Of course, the small samples in 2020 mean that most analyses regarding 2020 performance should be put in that context, and this is no exception. Generally, all players with significant gaps between their xwOBA and wOBA in 2020 are likely to see their wOBA move towards their xwOBA in 2021, and any trends that suggest such movement will be more or less severe should be taken with a grain of salt; those trends would be much more substantial had they been present over larger samples.

Even so, it’s worth looking into pitchers who either benefited from or struggled as a result of differences in xwOBA and wOBA in 2020. As a brief reminder, pitchers with a high xwOBA-wOBA likely benefited from factors out of their control (good luck) while pitchers with a low xwOBA-wOBA likely struggled as a result of factors out of their control (bad luck).

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Methodology

xwOBA-wOBA is generally used to quantify the effects of non-pitcher-controlled factors (like team defense and luck) on pitcher performance, and the metric does a pretty good job for the most part. It’s worth noting, though, that pitchers may have some control over their xwOBA-wOBA. 

Notably, pitchers who tend to induce ground balls at higher rates are likely to outperform their xwOBAs. Likely because shifts are more effective against ground balls and possibly because an infielder has a better chance of “luckily” fielding a ball or limiting damage than an outfielder, ground balls posted a 0.020 xwOBA-wOBA in 2020 compared to a -0.024 xwOBA-wOBA for non-ground balls.

With that (and the possibility of other xwOBA-wOBA biases) in mind, the leaderboards in the rest of this article will be based on deviations from each pitcher’s average xwOBA-wOBA from their past three seasons. It’s not perfect, but it should paint a clearer picture than just using raw 2020 xwOBA-wOBA numbers for determining which pitchers were most affected by factors outside of their control in 2020.

One more note before I get into player specifics: the leaderboards below include a “FIP Equivalent” field. xwOBA tends to correlate fairly strongly with FIP, and FIP has the advantage of being on the same scale as ERA. Using 2020 numbers, a one-point change in xwOBA is equivalent to about a 0.027 point change in FIP, and each xwOBA-wOBA gap will be stated in terms of FIP based on that scale in the “FIP Equivalent” field. The field shouldn’t be used as a hard measurement -- Dallas Keuchel’s FIP probably wouldn’t have been exactly 1.28 points higher if his wOBA had equaled his xwOBA in 2020 -- but instead only as a rough guide to contextualize the xwOBA-wOBA gaps.

Pitchers With The Highest xwOBA-wOBA

Below is a list of the 10 pitchers with the highest xwOBA-wOBA in 2020 (min. 150 PA) relative to their average xwOBA-wOBA from the last three seasons (min. 500 PA). The pitchers with asterisks next to their names are discussed in further detail below.

Dallas Keuchel

Keuchel has displayed a penchant for inducing desirable contact during his career thanks to a high ground-ball rate, but the 32-year-old’s 0.065 xwOBA-wOBA from 2020 is absurdly high, even for his standards. Keuchel has never posted an xwOBA-wOBA above 0.02 since Statcast began tracking the metric in 2015, and no pitcher (min. 450 PA) posted a mark above 0.045 in 2019.

To be sure, a solid White Sox defense that ranked second in the league by outs above average (OAA) and seventh by defensive runs saved likely helped Keuchel outperform his xwOBA, and the team’s defensive core should remain intact in 2021. But defensive metrics (and OAA in particular) tend to vary significantly from season to season, and that’s likely to be especially true heading into 2021 with the short 2020 season.

Defense aside, Keuchel’s 2.4% HR:FB was clearly unsustainable as well and is extremely unlikely to sit below 6% again next season. Additionally, Keuchel’s xwOBA-wOBA is likely to fall towards (and probably below) 0.02 next season, pushing his FIP up to around 3.80 even if he maintains his .310 xwOBA from 2020. It’s unlikely that Keuchel posts an ERA below 3.50 again next season as a result, and fantasy managers should temper their expectations after Keuchel’s spectacular 2020 season.

Rich Hill

You wouldn’t know it from Hill’s 3.03 ERA, but the 40-year-old had a major fastball problem in 2020. Despite seeing the pitch’s average velocity dip to 88 mph (his max velocity wasn’t much better at 91 mph) and its xwOBA jump from .283 to .399, the pitch’s wOBA rose only 31 points to .334. Given that Hill thew his fastball more than any other pitch in 2020 (46% usage rate), the divergence between his fastball’s underlying performance and actual results accounts for most of his xwOBA-wOBA gap.

Left without context, Hill’s poor underlying fastball performance suggests that he’ll struggle in 2021. But Hill missed chunks of the season with elbow and shoulder injuries, and he hadn’t been showing signs of fastball performance declines coming into the 2020 season. As a result, although Hill’s 0.063 xwOBA-xwOBA does not bode well for his future performance on its own, Hill should be expected to decrease his xwOBA substantially in 2021 (as long as he’s healthy), making his unusually high xwOBA-wOBA less concerning.

Brad Keller

Keller’s 0.082 xwOBA-wOBA is likely unsustainably high, but it came with some encouraging underlying trends. Like for Hill, Keller’s fastball is the key to his gap between xwOBA and wOBA as the pitch’s .344 xwOBA sat a long way from its .239 wOBA.  Under the hood, Keller’s fastball generated a 54% ground-ball rate in 2020, up 11 points from his 43% mark in 2019. 

The increased ground-ball rate isn’t enough to justify Keller’s fastball’s low wOBA on its own, though. As is fitting for a metric that seeks to eliminate factors out of a pitcher’s control from the equation, the Kansas City defense played a significant role in Keller’s (specifically Keller’s fastball’s) xwOBA being much higher than his wOBA. After shifting on only 23% of Keller’s plate appearances in 2019, the Royals shifted on a whopping 64% of Keller’s plate appearances in 2020 -- the 16th most in MLB. And if that’s not enough, the Royals shifted differently (likely in a more favorable way for Keller with the second baseman shifted further back more frequently) in 2020 compared to 2019 as well.

There are still caveats for Keller, though. Even with all of those changes factored into Keller’s performance, he probably won’t post such a pronounced xwOBA-wOBA next season. Additionally, it’s not clear how aggressively the Royals plan to employ shifts in 2021, and the general strength of the Kansas City defense is not something that fantasy managers should rely on. Furthermore, Keller’s fastball’s increased ground-ball rate does not appear to be related to a pitch location adjustment, so it’s not clear if that increase will stick in 2021. Still, Keller’s 0.082 xwOBA-wOBA is a lot less damning to his 2020 performance than it seems on its face, and an xwOBA-wOBA above 0.040 is not out of the question in 2021.

Zach Davies

Davies benefited from a low wOBA (compared to xwOBA) for each of his pitches in 2020, with his cutter’s .018 xwOBA-wOBA representing the lowest mark of the pitches in his arsenal. A strong San Diego defense that ranked first in the league in OAA by a wide margin likely played a significant role in Davies’ fortunate outcomes, making it difficult to expect him to outperform his career average 0.021 xwOBA-wOBA in 2021.

That’s not to say nothing is pointing in the right direction for Davies heading into next season. Although the Padres shifted on far fewer of Davies’ plate appearances in 2020 than the Brewers did in 2019, and the Padres were much more effective when they did shift, the team will probably be above-average defensively again in 2021 even if there is some drop-off in defensive effectiveness. Additionally, his emphasis on changeups at the expense of his sinker bodes well for his 2021 value. Still, Davies likely saw significant benefit from defense and luck in 2020, and fantasy managers should temper their expectations for 2021 as a result.

Pitchers With The Lowest xwOBA-wOBA

Below is a list of the 10 pitchers with the lowest xwOBA-wOBA in 2020 (min. 150 PA) relative to their average xwOBA-wOBA from the last three seasons (min. 500 PA). The pitchers with asterisks next to their names are discussed in further detail below.

Michael Wacha

Wacha endured a disastrous 2020 season that saw him post an ugly 6.62 ERA over 34 innings, but his ridiculous -0.079 xwOBA-wOBA suggests that bad luck played a significant role in Wacha’s struggles. To that end, Wacha allowed a career-worst and unsustainably high .366 BABIP last season, including a .325 BABIP on ground balls. Hitters posted a .700 OPS on ground balls against Wacha in 2020, a number that dwarfed the league average .491 OPS on ground balls.

A rebound for Wacha is likely in store for 2021 as a result, making the 29-year-old an intriguing fantasy option for later rounds in drafts. Wacha’s .312 xwOBA was in-line with his .313 mark from 2017 when he posted a solid but unspectacular 4.13 ERA, but Wacha also posted an 11.4% swinging-strike rate that represented his highest mark since the 2013 season thanks to increased changeup and cutter usage, providing him with added strikeout upside for 2021. That’s enough to make Wacha worth taking a chance on in 2021, though his role on whichever team he signs with will impact his fantasy value.

John Means

Means finished his rookie season in 2019 with a 3.60 ERA, a spot on the American League All-Star team, and a second-place finish in AL Rookie of the Year voting. It wasn’t a perfect rookie year (Means posted a lackluster 19% strikeout rate that contributed to a middling 4.41 FIP), but Means followed it up with a higher strikeout rate (23.9%), a lower walk rate (4%), and a lower xwOBAcon (.312) in 2020, fueling an impressive .262 xwOBA that tied Gerrit Cole for the 13th best mark among pitchers with at least 150 PA. Further working in Means’s favor is that those strikeout and walk rate changes were mostly well-supported by increased swinging-strike and zone rates, suggesting that his 2020 improvements may be here to stay, at least to some extent. 

Despite those improvements, Means saw his FIP increase to 5.60 in 2020 thanks to a 24.1% HR:FB that sat nearly six full points above the highest qualified pitcher’s mark. A higher than average HR:FB is probably inevitable for Means as long as he plays his home games at the homer-friendly Camden Yards, but it’s safe to expect Means’s HR:FB to fall significantly next season. Means is an excellent buy-low candidate heading into 2021 as a result, and fantasy managers should target the 27-year-old in drafts.

Trevor Richards

Richards’s .319 xwOBA from 2020 mirrored his 2019 mark, but the 27-year-old posted a ..370 wOBA in 2020 compared to a much more reasonable .316 wOBA in 2019. Home runs were one source of bad luck for Richards in 2020, as no pitcher (min. five home runs allowed) posted a lower average home run distance than Richards did (362 ft), and none of the home runs Richards allowed traveled more than 380 feet. Relatedly, Richards posted the second-lowest xwOBA on home runs (of the group) with a .755 mark but was tagged for a 1.94 wOBA (the value of a home run) for all six home runs he allowed.

Overall, it’s fair to expect Richards’s wOBA to fall back down close to his xwOBA again in 2021, and fantasy managers shouldn’t put much weight into Richards’s high 5.91 ERA from 2020. Richards still isn’t a particularly exciting fantasy option, but fantasy managers should keep the likely bad luck involved in his poor 2020 performance in mind heading into 2021.

Luke Weaver

Weaver’s 2020 struggles can largely be attributed to factors outside of his control, and his -0.046 xwOBA-wOBA supports that notion. Weaver posted identical .316 xwOBAs in 2019 and 2020, but his wOBA jumped from a fortunate .274 in 2019 to an unlucky .362 in 2020.

Given the small sample sizes from both seasons -- Weaver threw 64.1 innings in 2019 and 52 in 2020 -- and Weaver’s consistent xwOBA, it’s reasonable to expect Weaver to post an xwOBA around .316 again in 2021, and his wOBA should move significantly closer to his xwOBA. That xwOBA would put Weaver in line for a FIP around 4.50, and fantasy managers should expect Weaver to post an ERA around 4.50 in 2021 as a result.

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