@ccdad99: as I’m saying, HYPS would like, if they could, to be able to pick every individual in their classes and know with absolute certainty that if admitted, they will enroll - wouldn’t you, in their position?
It’s not enough for them to assemble “the same superstar class” from thousands of applicants that they don’t view as interchangeable. Their ultimate goal is to be sure that they’re getting this bassoonist rather than that one, exactly the desired numbers from exactly the right states, and on and on - and they’re not a million miles from being able to do that now, with yields of 70-80%.
To the extent they care about yield, this is the reason. As noted, they could fill their classes in the early round and have yields in the 90% zone, but they’d miss out on all the talent in the RD round, which they have a 60% or better chance of getting anyway if they admit them then.
And they know that the greatest barrier to their being able to hand-pick their classes is that they lose admits to each other. So they’ve deployed a tool (SCEA) that simultaneously (i) differentiates them from every other school that aspires to be grouped with them but relies on ED; and (ii) provides a high degree of assurance that the applicants they admit early will choose them, rather than any of the other members of the club. It’s quite clever, actually.
This is a world of difference from schools that have used ED as a defensive weapon to boost their yields so that they can (i) maintain their position in college rankings; and (ii) have at least some handle on who and how many of the kids they admit will actually show up, given that their RD yields are generally well below 50%.