The true value of xwOBA is the hot new debate in Sabermetric circles. Through the first two months of the 2018 season, what conclusions can we draw about the Pittsburgh Pirates from this metric?
The Pittsburgh Pirates’ run production has easily been the highlight of the 2018 season to date.
Even after taking into account the occasional bout of ineffectiveness that the club’s hitters are prone to, the results speak for themselves. As of this writing, heading into games played on May 24th, the Pirates’ team wRC+ of 109 ranks third in all of baseball (first in the National League). The team’s hitters carry the second-highest fWAR in the National League at 8.7. The club’s 19.2 percent strikeout rate is tops on the NL, and only .2 percentage points behind the MLB-leading Royals.
And, they are doing it with a modest xwOBA (Expected weighted on base average) of .338, good for 12th best in baseball.
Now might be a good time to define this shiny new Statcast statistic! xwOBA is defined by MLB as:
Expected weighted on-base average (xwOBA) is formulated using exit velocity and launch angle, two metrics measured by Statcast.
Feel free to hit that link above for the full formula. In the interest of brevity, I find it easy to explain away xwOBA as an improvement on BABIP (batting average on balls in play), in much the same way that wOBA is generally accepted as a huge improvement over batting average to measure full offensive contribution.
By isolating hit probability down to that which is within a pitcher and hitters’ control (launch angle and exit velo) while applying proper weights to different types of ways to get on base (balls weighted less, home runs weighted the most), xwOBA can succinctly explain to us what the results should say, or have said, while keeping it in the context of a single season.
The main issue that many take with xwOBA is the natural tendency to think of it as something predictive, rather than explanatory. A simple search for ‘xwOBA’ on Twitter will lead you to a ton of debate about its effectiveness. While most agree that the metric has utility, many caution against using it for prediction of any kind.
Having said that, what can xwOBA tell us about the Pittsburgh Pirates at the near two-month mark of the 2018 season?
Well-earned vs bad luck vs good luck
So if xwOBA is an improvement over BABIP, how “lucky” or “unlucky” are Pirates hitters? Let’s take a look:
(data as per Statcast)
Here, I’ve labelled the three biggest deductions from xwOBA in red, the three Pirates hitters outperforming their xwOBA in green, and those within a .010 percentage point range in yellow.
Those in yellow could be referred to as ‘wOBA true believers,’ those whose actual wOBA is very close to what it should be.
Is it any wonder that Corey Dickerson’s offensive output has been well-earned? He has easily been the most consistent run producer in the Pirates’ lineup. The man just refuses to strikeout, with an other-worldly 10.1 percent strikeout rate, fifth best among qualified MLB hitters.
Sean Rodriguez’s season has been underwhelming, yet his wOBA vs xwOBA tale is a tight one. This is likely due to eight of his 15 hits going for extra bases as well as his 15 walks. In this way, Rodriguez might be the poster boy for inherent flaws in xwOBA, as his .188 batting average (still relevant when it is that low!) and 31.6 percent strikeout rate would paint the picture of a milquetoast at best offensive threat.
Starling Marte, Elias Diaz and Josh Harrison all seem to be benefiting from “luck” with higher wOBAs than expected, but a closer look reveals there’s really nothing lucky about it.
In Marte’s case, a modest but significant improvement in walk rate — 8.6 percent this year against a 5.1 career rate — as well as a career mark in ISO at .195 has more to do with his high wOBA mark rather than any randomness. While Marte is not particularly hitting the ball that hard (just a 24.2 percent hard-hit rate as per FanGraphs), he is making the most “medium” rate contact with a 54.8 percent mark. So, yeah...walking more, better contact, less soft contact...a .368 wOBA makes perfect sense.
Diaz’s case is much the same. Carrying a 59.2 percent medium contact rate, a .407 on base percentage, four extra base hits and a 19-for-54 performance at the plate easily explains his wOBA. Harrison is a near opposite case than Marte and Diaz in that he has consistently made good contact with a 40.6 hard-hit rate as per Fangraphs. Harrison has been his usual selective self at the plate, with a 12.3 strikeout percentage and — like Marte — a modest improvement in walk rate (7.4 percent this year, up from a 4.0 percent career mark).
Harrison has always been wont to put the ball in play; the difference in 2018 is that he is making much better contact when he does.
What about those above in red? Surely something is happening to dampen their production?
Maybe. Maybe not.
Let’s start with the easy ones. David Freese and Jordy Mercer are not having great years in terms of BABIP. Freese clocks in at .280 while Mercer checks in with a .286 mark as of this writing. While xwOBA is meant to serve as an improvement on BABIP as outlined above, an out is still an out.
But, is it true bad luck? Actually, it might be.
Here, I have broken out the average exit velocity on Mercer and Freese’s non-strikeout, non-basepath outs and the exit velocity on their hits. I pulled the MLB-wide data for both, and it was interesting to see the duo outperforming MLB in both areas. Their difference in EV between a hit and an out is lesser than the baseball wide rate also. This tells me that they truly are being cursed with “bad luck,” though the degree to which is probably being overstated.
As for Francisco Cervelli...it’s scary to think what his season might look like had a few more balls went his way. A double here, a few more walks there, and an already fantastic season might be even scarier for opposing pitchers.
The Pittsburgh Pirates are barely through the first quarter of the 2018 season, but their run production to date has been formidable overall.
While xwOBA is not meant to be predictive, it would be worth revisiting this type of study as the season goes on to determine the true water level of this team’s offense.