A comparison of observational studies and randomized, controlled trials

By Kjell Benson, A. Hartz. American journal of ophthalmology-glaucoma, 2000

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Comparison of evidence of treatment effects in randomized and nonrandomized studies. (2001, background): “On the other hand, recent evaluations have suggested that for selected medical topics, both randomized and nonrandomized studies may yield very similar results.(7,8,13) There is a need to address these issues using empirical data from a large number of diverse medical topics.”
A checklist for retrospective database studies--report of the ISPOR Task Force on Retrospective Databases. (2003, result): “Two recent studies have suggested that adequately controlled observational studies produce results similar to randomized controlled trials [1, 2 ].”
Hierarchy of evidence: a framework for ranking evidence evaluating healthcare interventions. (2003, result): “Recently, however, comparisons of the results of observational studies and RCTs evaluating the same intervention have questioned this claim (Benson & Hartz, 2000; Concato et al., 2000), and suggest that the findings of observational studies are similar to those produced by RCTs.”