Community Medicine and Public Health: Measurement and Epidemiology
Community medicine rewards clean arithmetic and precise definitions. The examiner tests measures of disease frequency, validity of screening tests, study design and bias, plus national programme details and cold-chain rules.
Core rules
- Incidence counts new cases per population at risk per unit time; prevalence counts all existing cases at a point in time.
- Sensitivity = a/(a+c); specificity = d/(b+d); positive predictive value = a/(a+b), and it rises with prevalence.
- Cohort studies give relative risk; case-control studies give odds ratio.
- Herd immunity threshold approximates 1 minus 1/R0.
- Cold chain: OPV at minus 20 degrees Celsius, most other vaccines at 2-8 degrees Celsius.
Worked example
In a population of 1000, 100 people have the disease. A screening test is positive in 90 of the diseased and in 90 of the 900 healthy people. Sensitivity = 90/100 = 90 percent; specificity = 810/900 = 90 percent; PPV = 90/180 = 50 percent; NPV = 810/820 = 98.8 percent. At a 1 percent prevalence, the same test would give a PPV near 8 percent, showing why PPV cannot be transferred between settings.
Traps
- Quoting sensitivity as PPV, which is the commonest numerical error in the paper.
- Confusing lead-time bias with length bias in screening survival data.
- Forgetting that relative risk cannot be calculated directly from a case-control design.