A. Cohort study – Explanation
Whilst a case-control study may be used it would provide inferior evidence to that of a cohort study.
It is of course not ethical to give women placebo contraceptive pills, as would be required with a
randomised control trial
Study design
The following table highlights the main features of the main types of study:
| Randomized controlled trial |
Participants randomly allocated to intervention or control group (e.g. standard treatment or placebo) Practical or ethical problems may limit use |
| Cohort study | 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 |
| Case-control study | 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 |
| Cross-sectional survey | Provide a ‘snapshot’, sometimes called prevalence studies Provide weak evidence of cause and effect |
