John Snow's cholera investigation is a foundational example in epidemiology. What was the core reasoning that identified the Broad Street pump as the source?
AHe ran a randomized controlled trial comparing pump water to river water
BHe mapped disease distribution across persons and places and traced cases back to a common exposure
CHe applied Bradford Hill criteria to a large administrative dataset
DHe identified the Vibrio cholerae bacterium under a microscope
Snow mapped who got sick and where they lived, revealing that cases clustered around the Broad Street pump. This is pure epidemiologic logic: using the distribution of disease across person, place, and time to identify a likely cause. He didn't know the pathogen — his reasoning was distributional and causal, not microbiological.
Question 2 True / False
A study finds that people who carry lighters have higher rates of lung cancer. This correlation alone is sufficient to conclude that carrying lighters causes lung cancer.
TTrue
FFalse
Answer: False
Correlation does not establish causation. Carrying lighters is associated with smoking, which causes lung cancer — a classic confounding relationship. Epidemiology uses criteria like Bradford Hill (strength, consistency, temporality, plausibility, dose-response) to evaluate whether an association is likely causal, not correlation alone.
Question 3 Short Answer
Why does epidemiology define 'risk' at the population level rather than as a prediction for a specific individual?
Think about your answer, then reveal below.
Model answer: Risk is a probability estimated from observed rates in a defined population over a defined time window. It cannot tell you whether a specific individual will develop a disease — only that a person with certain characteristics faces a given probability. Individual outcomes are influenced by factors unmeasured in any study.
Epidemiologic risk is an aggregate measure: the proportion of a population that develops disease given exposure. Translating population risk to individual prediction requires strong assumptions and additional information. Conflating population risk with individual fate is a frequent misapplication of epidemiologic findings.