"All the health rankings are based solely on the results of peer assessment surveys sent to deans, other administrators and/or faculty at accredited degree programs or schools in each discipline. All schools surveyed in a discipline were sent the same number of surveys."
That's directly from the US News website. Their site also says that only 39% of programs responded to the surveys. Their medical school rankings (many of their other grad school rankings are based off similar criteria), on the other hand, is based on a slew of factors, including: peer assessment, assessment by residency directors, research activity, MCAT score, and GPA. Rankings for other disciplines also generally include career placement and salary data. The methodology isn't perfect, but it's obviously much better than the PT school methodology which merely comprises of a peer assessment score. I do happen to go to a top #10 school according to US News, but honestly the ranking didn't play even a small part in my decision to attend my respective program.