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The Right Study Design for the Right Study Question.

I n 2000, Concato and colleagues challenged the established consensus on the hierarchy of clinical research design by comparing the results of welldesigned observational studies with randomized controlled trials (RCT) of the same topic. In their study, they reported no overestimation of the magnitude of the association between exposure and outcomes of RCTs and well-designed observational studies.1 Other investigators have shown that effect estimates in observational studies are most similar to RCTs when similar exclusion criteria are used and prognostic factors accounted for.2 Despite these reports, RCTs hold the foremost position in the pyramid of evidence-based medicine. Meanwhile, the ultimate reproach of observational studies is their vulnerability to the effect of unrecognized confounding factors which may distort the results. Irrespective of the perceived hierarchy of evidence, the appropriate choice of study design should integrate consideration of research question, cost, access to data, required epidemiologic measures, and the level of existing evidence on the exposure-outcome relationship of interest. Although RCTs are considered as evidence of highest grade, they can be difficult to conduct or even impossible and unethical in some instances such as for various surgical interventions. Other shortcomings exist such as prohibitive cost when attempting to identify risk factors, treatment safety, and prognostic indicators, where observational studies are best suited. In addition, rigorously selected and monitored inclusion and exclusion criteria may limit external validity and make RCTs an inferior choice for prognostic model development. Moreover, RCTs are not an appropriate choice of study for specific research questions such as evaluating the causes of rare diseases which may be far more efficiently and effectively conducted through a retrospective casecontrol design, while cross-sectional studies may be the most suitable design for ecological, diagnostic, and screening studies. The field of interventional pulmonology (IP), a primarily procedural-based field, also faces some challenges in research design. The comparison of procedural interventions is sometimes unethical, blinding occasionally impossible and study outcomes may suffer from short follow-up periods because of limited patient survival. In this issue of the journal, Porcel et al3 present their experience over 5 and a half years of using indwelling tunneled pleural catheters (IPC) to treat recurrent pleural effusions. The authors attempt to sort out factors that may predict IPC removal, and IPC-related infection, using a single-center retrospective observational study design. A total of 308 patients receiving 336 IPCs were included in the study, majority of whom suffered from malignant pleural effusion (MPE) (83%). Pleurodesis occurred in 51% of the patients at a median