<p>Often, rankings are based on reputation of senior faculty who may or may not be very active in any given year. This ranking, on the other hand, measures how active/productive the professors are in a given year, regardless whether they are stars or not in their fields. The irony is the newbies/assistant profs are often the most productive because they tend to be the most ambitious or because they are pressured by the "published or perished" policy.</p>