BHS Epidemiology 3 — Questions and Answers
Question 1: Sensitivity of a screening test is defined as the probability of testing positive given that the person:
- Does not have the disease
- Has the disease (Correct answer)
- Has been vaccinated
- Is in the high-risk group
Correct answer: Has the disease
Sensitivity = true positives / (true positives + false negatives), measuring a test's ability to correctly identify those with disease.
Question 2: Which epidemiologic study design is most appropriate for studying a rare disease with a long latency period?
- Cross-sectional study
- Case-control study (Correct answer)
- Ecological study
- Open cohort study
Correct answer: Case-control study
Case-control studies are efficient for rare diseases because they select cases directly rather than waiting for them to occur in a large cohort.
Question 3: The number of secondary cases generated by one infectious case in a fully susceptible population is called:
- Herd immunity threshold
- Serial interval
- Basic reproduction number (R₀) (Correct answer)
- Force of infection
Correct answer: Basic reproduction number (R₀)
R₀ quantifies transmissibility; if R₀ > 1 the infection will spread, while R₀ < 1 leads to extinction.
Question 4: A dose-response relationship between an exposure and disease outcome is considered evidence of:
- Confounding
- Interaction
- Causal association (Correct answer)
- Ecological fallacy
Correct answer: Causal association
A dose-response gradient is one of Hill's criteria supporting a causal relationship between exposure and disease.
Question 5: Which of the following is NOT a criterion in Bradford Hill's guidelines for causality?
- Temporality
- Biological plausibility
- Statistical significance alone (Correct answer)
- Consistency
Correct answer: Statistical significance alone
Hill's criteria include temporality, strength, consistency, specificity, plausibility, coherence, experiment, and analogy — statistical significance alone is not a criterion.
Question 6: When a disease's frequency differs by geographic region but the individual-level association cannot be inferred from group-level data, this error is called:
- Survivorship bias
- Ecological fallacy (Correct answer)
- Neyman bias
- Lead-time bias
Correct answer: Ecological fallacy
The ecological fallacy occurs when group-level associations are incorrectly applied to individuals.
Question 7: A randomized controlled trial is considered the gold standard for establishing causality primarily because randomization:
- Eliminates all measurement error
- Controls for known and unknown confounders (Correct answer)
- Ensures blinding of participants
- Increases statistical power
Correct answer: Controls for known and unknown confounders
Random allocation distributes both measured and unmeasured confounders equally between groups, removing confounding as an alternative explanation.
BHS Epidemiology 3
Sensitivity of a screening test is defined as the probability of testing positive given that the person: