Epidemiological studies of the role of the suspected factor in the etiology of a disease may be observational or experimental. The essential difference between observational and experimental studies is that in observational studies:

A sample size of at least 3,000 should be used.

The investigators obtain data on exposures and outcomes naturally occurring without manipulating study factors such as the exposure variables. *

There is an assignment to treatment or exposure by the investigator.

The study and control groups are of equal size.