@FStratford an EA yield of at least 50% seems right and 75% is probably close to reality. Those EA kids are expressing a preference for UChicago over other schools but are more price sensitive. Some will get “sufficient” merit aid, others won’t. Some who don’t will still opt to attend once they’ve seen all their choices. But the ones who “get enough” would definitely attend. So that hints at something north of 50%.
For RD, those kids expressed a preference for another school first, and then chose not to apply EDII so clearly are as t least as price sensitive as the EA kids but probably more so. Also, they are the least committed to UChicago of the four segments (Two binding, Two non). Is it any surprise that the occasional snarky poster we’ve been seeing is an RD admit (rather than ED or EA). Probably weren’t given any money 
Merit packages for this group might well be reserved for the talented few but in general those who got an RD admit from HYPS will attend there, and those who got (more) merit aid at a slightly lower tier will attend that, instead of UChicago. That leaves something south of a 50% yield - guessing it’s 25% - 33% (so 30% seems about right).