Public Health and Epidemiology
Public health and epidemiology test the arithmetic of screening, study design, bias, and the structure of the Israeli health system.
Core rules
- Incidence counts new cases; prevalence counts all existing cases. Prevalence rises with longer survival and falls with faster cure.
- Sensitivity and specificity belong to the test; positive and negative predictive values depend on prevalence.
- Cohort studies give relative risk; case-control studies give odds ratios.
- Common biases: selection, recall, lead-time, length-time, confounding.
- Outbreak investigation: define the case, count, describe by time/place/person, form a hypothesis, test it, control.
- Israel: the National Health Insurance Law (1994) guarantees coverage through four health funds - Clalit, Maccabi, Meuhedet and Leumit - with a basket updated annually by a public committee.
Worked example
A test has 90% sensitivity and 95% specificity; prevalence is 1% in 10,000 people. Diseased: 100, of whom 90 test positive. Healthy: 9,900, of whom 495 are false positives. PPV = 90/585 ≈ 15%. Reasoning: most positives are false, so a positive result needs confirmation. Raising prevalence raises PPV without changing sensitivity or specificity.
Traps that cost marks
- Confusing relative risk with odds ratio.
- Quoting PPV as if it were fixed.
- Calling a cross-sectional association causal.
- Confusing lead-time bias with a genuine survival benefit.