The XI Corps was a badly organized and badly led corps in the Union Army. It bore the brunt of Confederate Stonewall Jackson's successful attack at Chancellorsville, and was also heavily defeated on the first day of the Battle of Gettysburg.
On the second and third days of the Battle of Gettysburg, southern General Robert E. Lee ordered unsuccessful attacks on the Union III Corp at Devil's Den, V Corp on Little Round Top, Pickett's Charge against the II Corp on Cemetery Ridge.
Although badly generaled by Daniel Sickles, the III Corp was well-led at the divisional level and below. The V Corp contained some stout regiments, notably the 83rd Pennsylvania, and the above all, the 20th Maine, which was immortalized in the "Killer Angels (book and movie) for its defense of Little Round Top. And the II Corp was apparently the Union Army's version of General Patton's Third Army from World War II (that is, it suffered and inflicted the most casualties of any unit its size).
Which was more "typical" of the Union Army, the XI Corps or the other three? More to the point, can one reasonably believe that General Lee's mistaken moves at Gettysburg may been predicated on the assumption that the XI Corp was "representative" of the rest of the Union army when in fact it might not have been? Put another way, would Lee's tactics have made more sense if the attacks had been directed at replicas of the XI Corp rather than the three units that actually received them?
Edit for background: I didn't want to go into "legacy issues" earlier, but this question was inspired by a comment made on one my previous questions: "@RISwampYankee: There were certainly crack Union units, but the weakest Confederate units were nearly on par with average Union units until late in the war..." Pieter Geerkens Sep 11 at 3:14"
I believe that the "average" Union unit was weaker than the "average" Confederate unit, only because a few units pulled down the "mean," and that the median Union unit was actually as good as the median Confederate unit. Unless Pieter was asserting only that the quality of Union units was more "skewed" than Confederate units, I disagree with the above, and was trying to find out if the "typical" Union unit wasn't a lot better than the least successful one.