Understanding Study Design
Clinical research uses a wide array of study designs. In observational studies, also known as nonexperimental studies, investigators evaluate characteristics, behaviors, and exposures in participants with a particular disease, condition, or complication. An observational study reports only the characteristics of the study population; it does not directly manipulate behaviors (eg, cigarette smoking) or exposures (eg, use of a medication, laser treatment). In experimental studies, typically clinical trials, subjects are assigned to a particular treatment, such as a prescribed behavior (eg, eating a diet high in antioxidant foods), or a therapeutic or preventive intervention (eg, use of an oral neuroprotective agent for patients with glaucoma, antioxidant vitamin supplementation for patients with early age-related macular degeneration [AMD]).
When conducted and interpreted appropriately, each type of study design may provide valuable information. Researchers employ observational studies when describing the presentation and progression of disease, generating hypotheses, and efficiently assessing data that may already exist for testing a hypothesis about an intervention. Examples of observational studies include case reports, case series, case-control studies, cross-sectional studies, and cohort studies. In contrast, prospective randomized controlled trials provide the best evidence regarding the effects of an intervention. Finally, meta-analyses provide a methodology to summarize the results of multiple clinical trials addressing similar research questions. Figure 1-2 depicts the levels of evidence that can be obtained from different study designs; note that meta-analyses and controlled trials offer the highest levels of evidence.
Figure 1-2 The pyramid of evidence, which illustrates the relative strength of different study designs.
A case report describes a finding in regard to a single patient to alert readers to a rare condition or unusual treatment result. For example, in 2005, Friedman reported retinal vasculitis in an apparently healthy patient with none of the common causes of vasculitis, such as toxoplasmosis, syphilis, Behçet disease, sarcoidosis, lupus, or herpes. A magnetic resonance imaging (MRI) scan of the patient’s brain revealed findings typical of multiple sclerosis (MS). Although clinicians recognize that retinal vasculitis develops in MS patients, this study was the first to report retinal vasculitis as the initial presentation of MS. This case report demonstrated that clinicians should consider MS when they have tested for more common causes of vasculitis, but the etiology remains unclear.
Case reports cannot provide information on treatment efficacy or assert whether a disease is caused by an exposure. At most, they can suggest a previously unsuspected finding or mechanism of disease.
Excerpted from BCSC 2020-2021 series: Section 1 - Update on General Medicine. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.