Primary data sources, by definition, are clinical studies designed to generate new information. Different types of studies exist to meet various research objectives, making them valuable tools in outcomes research (OR). Randomized-controlled trials (RCTs) are regarded as the gold standard of experimental studies due to their high internal validity. This makes them particularly suitable for the development phase of new pharmaceuticals, where demonstrating efficacy and safety for marketing authorization is crucial. However, strict patient management, rigorous inclusion and exclusion criteria, and the use of surrogate parameters can limit their use in OR, and results might not reflect outcomes in everyday clinical practice. Observational studies, such as post-marketing surveillance studies, rank below RCTs on the evidence hierarchy but can offer important insights into real-world effectiveness. These studies assess how treatments perform under routine conditions and capture safety data from broader and more diverse populations. Among observational studies, prospective cohort studies are particularly useful for examining how risk factors influence health outcomes over time. Cross-sectional studies, by contrast, provide a "snapshot" of health conditions, risk factors, or disease prevalence at a single point in time.