Community Medicine and Public Health
Core framework
Community medicine questions test epidemiology, biostatistics, health program design, and the social determinants of health. Master measures of disease frequency (incidence versus prevalence), measures of association (relative risk, odds ratio, attributable risk), study designs and their biases, screening test properties (sensitivity, specificity, predictive values), outbreak investigation steps, and the structure of national health programs including maternal and child health and immunization.
Concepts you must own
Incidence measures new cases and reflects risk; prevalence measures existing cases and reflects burden. Sensitivity and specificity are properties of the test; predictive values depend on prevalence. Relative risk comes from cohort studies; odds ratio from case–control studies. Bias types: selection, information, and confounding.
Traps
- Prevalence versus incidence in a stem about duration or burden.
- Predictive value changes with prevalence even when sensitivity and specificity are fixed.
- Odds ratio approximates relative risk only when the outcome is rare.
Worked rule of thumb
For a 2×2 table, compute sensitivity as true positives divided by all diseased, and specificity as true negatives divided by all non-diseased. Then compute positive predictive value as true positives divided by all test positives. If prevalence in the population falls, positive predictive value falls while sensitivity and specificity stay constant. This single relationship answers most screening questions.