Why are applications for 3rd and 4th rounds believed to be so difficult?
I would have thought that most of the super star applicants would have had their *deleted* together by the early rounds and there would be less of them competing for spots in the later rounds.
Well thats the catch isn't it - since most of the "strong" applicants apply in both R1 and R2, you are going to want to get your classes filled with primarily R1 and R2 candidates, because you know that the R3 and R4 (if applicable) candidates will be relatively speaking, on average, weaker.
So, to make sure you get the top people, you try to fill most of your class in R1/R2. Say for instance you wanted to fill a class of 450 or so and you had a 50% yield. You might make 400 offers in R1, 400 offers in R2, and 100 in R3. I'm overdoing it a bit - its probably more like 300, 300, 200 or something like that, but the idea is that. Yields vary very very little from year to year really (at my alma mater, they've changed by less than 0.5% over the last 3 years, despite an application volume increase of nearly 25%).