Community Medicine and Public Health: Populations, Screening and Epidemiology
Community Medicine and Public Health: what is tested
This domain tests population measures, study design, screening, outbreak investigation and health programme indicators. The exam favours calculation and interpretation over definitions alone.
Core rules
- Incidence counts new cases per population at risk per unit time; prevalence counts all existing cases and rises with longer duration. Prevalence approximately equals incidence x duration for stable diseases.
- Sensitivity = true positives / all diseased; specificity = true negatives / all non-diseased. Positive predictive value = true positives / all test positives and depends on prevalence.
- Cohort studies give relative risk; case-control studies give odds ratio. Bias includes selection, information and recall bias; confounding is a separate variable linked to both exposure and outcome.
- Screening is worthwhile when the disease is important, detectable early, treatable and the test is acceptable and valid. Outbreak investigation starts with case definition, case finding, descriptive epidemiology, hypothesis testing and control measures.
Worked example. In a population of 1,000, 100 have disease. A test is positive in 80 diseased and 90 non-diseased people. Reasoning: sensitivity = 80/100 = 80%; specificity = 810/900 = 90%; PPV = 80/(80 + 90) = 47.1%; NPV = 810/830 = 97.6%. If prevalence fell, PPV would fall while sensitivity and specificity stayed the same.
Traps that cost marks
- Reporting prevalence when the question asks for incidence.
- Using PPV as a fixed test property independent of prevalence.
- Confusing relative risk with odds ratio in a case-control study.
- Forgetting that a screening test needs an acceptable false-positive burden.