Observational research methods. Research design II: cohort, cross sectional, and case-control studies
This article from the BMJ describes "Cohort, cross sectional, and case-control studies...collectively referred to as observational studies. Often these studies are the only practicable method of studying various problems, for example, studies of aetiology, instances where a randomised controlled trial might be unethical, or if the condition to be studied is rare. Cohort studies are used to study incidence, causes, and prognosis. Because they measure events in chronological order they can be used to distinguish between cause and effect."