Community Medicine (Preventive and Social Medicine)
What is tested
PSM rewards measurement: study designs, screening statistics, national programme targets, demography and environmental health. Numeric questions are frequent and quick to score.
Core rules
- Incidence vs prevalence: incidence counts new cases per population at risk per unit time; prevalence counts all existing cases. Prevalence ≈ incidence × duration.
- Diagnostic statistics: sensitivity = TP/(TP+FN); specificity = TN/(TN+FP); PPV = TP/(TP+FP). PPV rises with prevalence; sensitivity and specificity do not.
- Study design: relative risk comes from cohort studies, odds ratio from case-control studies, incidence from cohort studies, prevalence from cross-sectional studies.
- Health infrastructure: sub-centre 1 per 5,000 rural (3,000 hilly); PHC 1 per 30,000 rural (20,000 hilly); CHC 1 per 120,000 (80,000 hilly).
- Screening biases: lead-time bias (earlier diagnosis without longer survival), length bias (slow disease over-represented), selection bias.
Worked example
In a 2×2 table, 90 of 100 diseased and 20 of 200 non-diseased test positive. Sensitivity = 90/100 = 90%. Specificity = 180/200 = 90%. PPV = 90/110 = 82%. If prevalence fell to 1%, PPV would fall to about 8% while sensitivity and specificity stayed at 90% - the reason screening targets high-risk groups.
Traps
Confusing incidence with prevalence; computing PPV as though it were sensitivity; and using an odds ratio as a direct risk estimate in a common disease.