Community Medicine and Public Health
What you must understand
Community medicine questions test epidemiology, biostatistics, health programmes and prevention. Know the measures of disease frequency (incidence vs prevalence), association (relative risk, odds ratio), and screening (sensitivity, specificity, predictive values). Understand study designs (cohort, case-control, cross-sectional, randomised trial) and their biases. Learn the levels of prevention (primordial, primary, secondary, tertiary) and the structure of national health programmes.
Traps that appear in questions
- Confusing incidence (new cases over time) with prevalence (all existing cases at a point).
- Mixing up sensitivity (detects true positives) with specificity (detects true negatives).
- Forgetting that predictive values depend on prevalence, while sensitivity and specificity do not.
- Overlooking confounding and selection bias in study-design questions.
Worked example (rule of thumb)
A screening test with 90% sensitivity and 95% specificity in a low-prevalence population will have a low positive predictive value - most positives are false positives. Rule of thumb: low prevalence lowers PPV even for a good test.
Exam strategy
For epidemiology stems, draw a 2×2 table and compute the measure asked. For programme questions, link the intervention to the level of prevention.