Clinical trials

  • Parallel randomised
  • Parallel non-randomised
  • Cross-over

Community trials

Historical controlled trials

Cohort study

  • Prospective
  • Retrospective



Observational Studies

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. 

Cross-sectional (prevalence) study: evaluates the prevalence of exposures and outcomes at the same time. Temporal association is difficult to establish. 

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.  

Prospective cohort study: follows and assesses outcomes in exposed and unexposed groups over time. 

Retrospective cohort study identifies populations with and without the exposure based on past records and assesses outcomes at the time of study. At greater risk of selection and information bias. 


Investigator assigns exposures. 

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.