Community Medicine and Public Health
What you must understand
Community medicine tests measurement and decision-making at population level. Know the definitions of incidence and prevalence, and when each is appropriate; understand sensitivity, specificity, predictive values and how prevalence changes predictive values. Learn the design, strengths and weaknesses of cohort, case–control and cross-sectional studies, and the meaning of bias, confounding and effect modification. Understand measures of association - relative risk, odds ratio, attributable risk - and the criteria for causal inference.
Master the principles of screening and its criteria for a valid screening programme. Know the levels of prevention and give an example of each. Understand the components of the national immunisation schedule, cold chain, and disease surveillance. Learn water purification, sanitation and vector control measures, and the classification of occupational and environmental hazards. Know the structure of the health system and the indicators used to monitor maternal and child health.
Traps that appear
Questions often confuse incidence with prevalence, or ask for predictive value when the candidate computes sensitivity. Remember that sensitivity and specificity are properties of the test, while predictive values depend on prevalence. Distinguish crude from standardised rates. A common trap is presenting a study design that cannot measure incidence - a case–control study cannot directly yield incidence.
Worked rule of thumb
For any screening question, apply the five criteria: important disease, accepted treatment, detectable early stage, suitable test, and cost-effective programme. For study design, ask whether exposure or outcome was the starting point; if exposure, it is cohort; if outcome, it is case–control.
Study strategy
Practise calculating a two-by-two table from a short vignette until sensitivity, specificity and predictive values are automatic.