Community Health and Public Health: Measurement and Control
What you must know
Distinguish incidence (new cases over a period) from prevalence (all existing cases at a point in time), and rate from ratio. Understand sensitivity, specificity, predictive values, and how prevalence alters the positive predictive value. Know the levels of prevention, the criteria for a good screening test, the steps of an outbreak investigation, and the building blocks of a health system.
Classic traps
- A highly sensitive test rules disease out (SnNout); a highly specific test rules disease in (SpPin).
- Predictive values depend on prevalence; sensitivity and specificity do not.
- Maternal mortality ratio is per 100,000 live births; the maternal mortality rate uses women of reproductive age as the denominator.
- Coverage and utilisation differ, and a cold chain failure does not always destroy vaccine potency; read the vaccine vial monitor.
Rule of thumb
In an outbreak: verify the diagnosis, confirm the outbreak exists, define and find cases, describe by person, place and time, generate hypotheses, test them, institute control, communicate, and write up. When judging a screening programme, ask whether early detection actually changes outcome, because lead-time bias can make an ineffective test look beneficial.