Community Health and Research Principles
Population thinking
Community health asks you to move from one patient to a population. Know the measures: prevalence is all existing cases at a point in time, while incidence is new cases over a period. Indices quantify disease - DMFT/DMFS for caries, PSR/CPITN for periodontal screening, OHI-S for oral hygiene, and fluorosis indices. Prevention levels: primary (fluoride, sealants, education), secondary (screening, early lesion management), tertiary (rehabilitation, prosthetics). Program planning follows assessment, goals and measurable objectives, implementation, and evaluation.
Study designs
Cohort studies follow exposed and unexposed groups forward, yielding incidence and relative risk. Case-control studies start with disease and look backward, yielding odds ratios. Cross-sectional surveys give prevalence and associations but not causality. Randomized controlled trials best support causal inference. Traps: confusing relative risk with attributable risk, treating a confidence interval that crosses 1.0 as significant, and forgetting that predictive value depends on prevalence.
Worked example
Screen 1,000 people. Of 90 with caries, 60 test positive; of 910 without caries, 100 test positive. Sensitivity = 60/90 = 66.7%. Specificity = 810/910 = 89.0%. Positive predictive value = 60/160 = 37.5%. Negative predictive value = 810/840 = 96.4%. Note that PPV falls as prevalence falls even when sensitivity and specificity are unchanged - a common exam trap.
Rule of thumb: sensitivity describes the diseased, specificity describes the healthy, and predictive values describe what a result means in your population.