Historical controlled trials
Investigator does not assign exposures. A descriptive study provides a description of exposures and outcomes, whereas an analytic study provides a measure of the association between exposure and outcome. Observational studies are hypothesis generating and cannot establish causal association.
Case-control study: compares subjects with disease (cases) to those without disease (controls) for risk factors. It is useful for establishing hypotheses about aetiology, especially when a disease is rare. It cannot estimate the prevalence of disease. Controls and cases should be sufficiently similar in prognostic factors other than the risks of interest.
Before and after (pre-post) observational study: outcomes are measured before and after an exposure, that is not assigned by the investigator. Study participants in the pre- and post-periods may be the same or different.
Cross-sectional (prevalence) study: evaluates the prevalence of exposures and outcomes at the same time. Temporal association is difficult to establish.
Investigator assigns exposures (treatments).
Clinical trial: subjects with disease are placed in different treatment groups. The study population must be sufficiently similar and representative of the general population of patients. The intervention is compared to a placebo, no treatment or an alternative treatment.
Community trial: groups of subjects are assigned to different treatments.
Field trial: subjects without disease are placed in different preventative intervention groups.