<p>I believe that the basic divisions are based, as nngmm says, on USN&WR rankings.</p>
<p>At its simplest, the tiers would be:</p>
<p>Rank 1-50 First Tier
Rank 51-100 Second Tier
Rank 101-150 Third Tier (but listed alphabetically without individual ordering within the tier)
Rank 151-200 Fourth Tier (same listing approach as Tier 3).</p>
<p>I believe the difference in the numbers for each tier that I have listed (50 per tier) and the “odd” numbers nngmm has listed is that there are a number of ties in the rankings.</p>