Critically Reviewing the Literature for Improving Clinical Practice

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Various hierarchies have been proposed for classifying study design. In simplest terms, studies can be classified as case series, case control studies, cohort studies, and randomized controlled trials (RCTs). The case series is the weakest and the RCT is the strongest for determining the effectiveness of treatment (Table 54.1). Case reports (arbitrarily defined as 10 or fewer subjects) and case series are the typical surgical studies performed. There is no concurrent control group although there may be a historical control group. Patients may be followed from the same inception point and followed prospectively – not for the purpose of the study – but in the normal clinical course of the disease. Typically, data from patient charts or clinical databases are reviewed retrospectively. Thus, the outcome of interest is present when the study is initiated. However, results from case series should be likened to those observations made in the laboratory. Just as those observations should lead to generation of a hypothesis and performance of an experiment to test it, an RCT should be performed to confirm the observations reported in a case series. Case series are plagued with biases such as selection and referral biases, and because data are not collected specifically for the study, they are often incomplete or even inaccurate. Therefore, incorrect conclusions about the efficacy of a treatment are common and surgeons should not rely solely on evidence from case series.