The epidemiological studies are divided into two broad categories: Observational and experimental studies.

Observational studies are sometimes called a natural experiment. We the researchers only observe the natural development of the studies without any manipulation of the independent variables. Whereas in experimental studies the researchers can manipulate the variables.

Observational studies are again divided into descriptive (such as case report, case series, cross sectional or ecological studies) and analytical (such as case control and cohort studies).

1. State the main differences between observational and experimental studies