Community Medicine and Epidemiology: measuring risk in populations
Scope
This domain tests epidemiology (measures of disease frequency and association, study designs, bias, outbreak investigation), screening, health services and primary health care, environmental and occupational health, and maternal and child health programmes.
Core rules
- Incidence measures new cases per population at risk per unit time; prevalence measures all existing cases. Prevalence is approximately incidence multiplied by duration.
- Relative risk = incidence in exposed ÷ incidence in unexposed. Odds ratio approximates RR only when the disease is rare.
- Attributable risk = incidence in exposed − incidence in unexposed; it measures excess risk among the exposed.
- Screening tests need high sensitivity to exclude disease; confirmation needs high specificity.
Worked example
In a cohort, 20 of 100 exposed and 20 of 200 unexposed develop disease. Incidence in exposed = 0.20; in unexposed = 0.10. RR = 0.20/0.10 = 2.0, so exposure doubles risk. OR = (20 × 180)/(80 × 20) = 2.25. Because the disease is common here, the OR overstates the RR and the RR should be reported.
Traps that cost marks
- Using prevalence as if it were a risk in a cohort study.
- Claiming a screening test is accurate from sensitivity alone; predictive values depend on prevalence.
- Confusing confounding, a third variable linked to both exposure and outcome, with bias in selection or measurement.
- Forgetting that a cross-sectional study measures association but cannot establish temporality.