We read with great interest the editorial comment regarding our recent publication RL after RP. We, along with other investigators who have examine the postoperative outcomes using similar cohorts,1'5 recognize the inherent limitations of retrospective observational cohorts. However, one should wonder why, despite such limitations, numerous well-designed systematic studies using large population-based cohorts exist? Are all such studies without any clinical value? On the contrary, population-based cohorts come with a number of advantages that institutional databases lack. For one, population-based cohorts generally benefit from a larger number of patients than institutional cohorts. Also, population-based cohorts are composed of a more heterogeneous group of individuals who come from many different hospitals, which can include both teaching and nonteaching centers, situated at various regions throughout the United States. Such characteristics allow clinicians and research scientists to attest whether the findings of select referral centers also apply to the general population. In that regard, studies relying on population-based cohorts play an important role in systematic research, which seeks to address a specific research topic and to test a hypothesis. As such, findings thatoriginate only from population-based cohorts are also insufficient. In fact, both observational and institutional studies, along with prospective randomized trials, are needed to establish the generalizability and validity of any findings.
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