Quantitative Research Designs

ExperimentalObservational: Analytic 

Clinical trials

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

Community trials

Historical controlled trials

Prospective cohort study

  • Concurrent
  • Non-concurrent



Observational Studies

Observational study: who, where, when. 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. 

Ecologic study: provides a description of population group characteristics. 

Cross-sectional study: evaluates the association between exposures and outcomes at the same time. It allows measurement of  the prevalence of exposure or outcome. Temporal association is more difficult to establish. 

Case-control study: compares subjects with disease (cases) to those without disease (controls) for differences in risk factors. It is useful for establishing hypotheses about risk factors and aetiology, especially when a disease is rare. It cannot estimate the prevalence of disease. Controls and cases should be sufficiently similar in prognostic factors unrelated to the risk of interest. 

Cohort study: follows and assesses outcomes in exposed and unexposed groups over time. A retrospective cohort study identifies populations with and without the exposure based on past records and assesses outcomes at the time of study. Common biases are selection bias and information bias. 


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.   


Essentials of Clinical Research Design (Pediatr Invest 2019)