Community Health and Research Principles
What the exam tests
This domain combines epidemiology, biostatistics, research design, and community program planning. Know measures of disease frequency (incidence, prevalence, DMFT/dmft, community periodontal index), study designs (cross-sectional, case-control, cohort, randomized controlled trial), measures of association (relative risk from cohort studies, odds ratio from case-control studies), screening test properties (sensitivity, specificity, predictive values), reliability versus validity, levels of prevention, and the evidence base for community water fluoridation. Basic statistics, including measures of central tendency and dispersion, sampling methods, and bias, are also fair game.
Traps that recur
- Incidence versus prevalence. Incidence counts new cases over time; prevalence counts all existing cases at a point. Prevalence approximates incidence multiplied by duration, so longer survival raises prevalence without raising incidence.
- Sensitivity versus specificity. Sensitivity is the proportion of diseased people who test positive; specificity is the proportion of nondiseased people who test negative. Predictive values depend on disease prevalence in the population; sensitivity and specificity do not.
- Design and association. Cohort studies yield relative risk and attributable risk; case-control studies yield only an odds ratio.
- Prevention levels. Primary: fluoride, sealants, education. Secondary: screening and early detection. Tertiary: rehabilitation and limiting disability.
- p-values. A p-value is not the probability the hypothesis is true, and it says nothing about effect size.
Worked example
Of 100 screened people, 40 have the disease and 32 of them test positive, so sensitivity is 32/40 = 80%. Of the 60 without disease, 48 test negative, so specificity is 48/60 = 80%. Positive predictive value is 32/44, about 73%.