D. Case-control study – Explanation
As the outcome (childhood leukemia) is relatively rare a cohort study would take an extremely long
time to provide significant results
Study design
The following table highlights the main features of the main types of study
| Randomised 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. Groups selected based on exposure to an agent (e.g., medicine, toxin) and followed to see if they develop a disease. Outcome measure: relative risk. Example: Framingham Heart Study. |
| Case-control study | Observational and retrospective. Patients with a condition (cases) are matched with controls. Past exposure to a possible causal agent is collected. Outcome measure: odds ratio. Inexpensive, quick results, useful for rare conditions, but prone to confounding. |
| Cross-sectional survey | Provides a ‘snapshot,’ also known as prevalence studies. Provides weak evidence of cause and effect. |
