Study Design

Question 1

Participants randomly allocated to intervention or control group (e.g. standard treatment or placebo)

Practical or ethical problems may limit use

Answer 1

Randomised Control Trial

Question 2

Observational and prospective. Two (or more) are selected according to their exposure to a particular agent (e.g. medicine, toxin) and followed up to see how many develop a disease or other outcome.

The usual outcome measure is the relative risk.

Examples include Framingham Heart Study

Answer 2

Cohort Study

Question 3

Observational and retrospective. Patients with a particular condition (cases) are identified and matched with controls. Data is then collected on past exposure to a possible causal agent for the condition.

The usual outcome measure is the odds ratio.

Inexpensive, produce quick results
Useful for studying rare conditions
Prone to confounding

Answer 3

Case-control Study

Question 4

Provide a ‘snapshot’, sometimes called prevalence studies

Provide weak evidence of cause and effect

Answer 4

Cross-sectional survey