Observational research in rheumatic disorders

Observational research plays an important role in science. Much of human knowledge, including health knowledge, comes from observations, not experiments. Within medicine, observational studies cover a wide scope that ranges from case reports and case series, to descriptive studies, to analytic observational studies for causal questions. These studies often are complementary; each provides a piece of information that can be synthesized into a description and analysis of a health problem in a way that tests hypotheses and advances medical science and has the potential to lead to interventions that can improve health and quality of life. This article reviews the scope of observational studies, principles of causal inference in observational studies, and various types of observational studies with a focus of their applications in rheumatic disorders.