Public Health and Epidemiology
Core concepts
Understand incidence and prevalence, and how they differ: incidence measures new cases over a period, while prevalence measures all existing cases at a point in time. Know measures of association, including relative risk and odds ratio, and the meaning of attributable risk. Be able to describe study designs, screening principles, and levels of prevention (primary, secondary, tertiary). Familiarity with outbreak investigation, herd immunity, and surveillance systems is expected.
Traps
A common error is using prevalence to estimate risk in a cohort; incidence is the appropriate measure when following people over time. Another trap is confusing relative risk with absolute risk: a doubled risk of a rare outcome may still be a small absolute increase. Questions may also test whether a screening test's positive predictive value changes with prevalence; it does, even when sensitivity and specificity stay constant.
Worked example
If 100 new cases occur among 2,000 people followed for one year, incidence is 100/2,000 = 0.05, or 5 per 100 person-years. Rule of thumb: incidence has a denominator of people at risk and a time frame; prevalence has no time dimension in its denominator.
Study tip
For any epidemiology question, identify the numerator, denominator, and time frame before choosing a formula.