Hold the horses here! Stories did an MPH at BU, right? But he/she got into Yale PhD's track in Epi or something. BU sure ain't ivy league, their public health school is like #12 BUT the percentage of those accepted who go there is lower than at other schools, meaning that you have a good shot of going there, so I don't think it is nearly as selective as Hopkins or Harvard. A lot of people I know used BU as a safety school and didn't go there, didn't think that it would as easily open up doors like UNC or a place like that?
So is Stories saying that his/her undergrad was Harvard and that is how he/she got into Yale?
I think that it matters if you go to Harvard, Hopkins, UNC, and others in the top 8 or so. Even then, if you do a good job at lower ranked places then you have a shot at going to a prestigious school.
(I actually got a 3.9 from a state undergrad and decided to go to JHU over other schools like BU, so don't tell me it can't happen!)
In terms of ranking, I think it is hard for the important ranking info not to be subjective. Two schools may have the same funding, same classes, but the faculty could have better interaction with the students at one of the schools, and impart to them world class problem solving strategies, while disinterested faculty produce students who know some facts, but not much else. This is why a lot of leaders in public health have come from Harvard, Hopkins, UNC, and others in at least the top ten, because people know that these students got a good education in terms of quality.
Another factor is motivation, if you are motivated to go to Hopkins or Harvard, then you are probably gung-ho public health and will apply to the more selective PhD programs later on, if you so choose.