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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.  

  • Nested case-control study: defines a cohort with suspected risk factors and a control is assigned for each case from within the cohort. Cases and controls are matched on calendar time and length of follow-up. 
  • Case-cohort study: cases are not matched on calendar time or length of follow-up to controls. 

Prospective cohort Cohort study:

  • Prospective follows and assesses outcomes in exposed and unexposed groups over time. 
  • Retrospective

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  • identifies populations with and without the exposure based on past records and assesses outcomes at the time of study

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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. 

  • Controlled before and after: the before-after effect of implementation in the intervention group is compared with a control group that has no intervention.

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

Experimental

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. 

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