Here are my tier definitions:

First Tier: top 15 programs
Second Tier: programs 16-25
Third Tier: programs 26-50

The first-tier programs are either elite in both phases or so astronomically elite in one phase that it makes up for being non-elite in the other. The second-tier programs are either excellent in both phases or elite in one phase to the point that it makes up for being non-elite in the other. The third-tier programs are usually either excellent in one phase or the other, but rarely in both, or they are simply consistently very good in both phases.