NBME Biostatistics and Epidemiology 2 — Questions and Answers
Question 1: Which measure of association is used in a case-control study?
- Relative risk
- Odds ratio (Correct answer)
- Attributable risk percent
- Incidence rate ratio
Correct answer: Odds ratio
Because case-control studies start with outcome status (not exposure), incidence cannot be calculated directly; the odds ratio is used as an approximation of relative risk.
Question 2: A disease has a prevalence of 50 per 1,000 and an incidence of 10 per 1,000 per year. What is the average disease duration?
- 0.2 years
- 5 years (Correct answer)
- 10 years
- 50 years
Correct answer: 5 years
Using the steady-state formula Prevalence = Incidence × Duration: Duration = 50/10 = 5 years.
Question 3: Which study design provides the STRONGEST evidence for a causal relationship between an exposure and outcome?
- Case-control study
- Prospective cohort study
- Cross-sectional study
- Randomized controlled trial (Correct answer)
Correct answer: Randomized controlled trial
Randomized controlled trials eliminate confounding through random assignment, providing the highest level of evidence for causality.
Question 4: Berkson's bias is a specific type of selection bias most likely to occur in:
- Prospective cohort studies
- Hospital-based case-control studies (Correct answer)
- Randomized controlled trials
- Population-based cross-sectional surveys
Correct answer: Hospital-based case-control studies
Berkson's bias arises when hospitalized controls differ systematically from the general population, distorting the exposure-disease association in hospital-based case-control studies.
Question 5: The attack rate during an outbreak investigation is calculated as:
- (Number of new cases / Total population at risk) × 100 (Correct answer)
- (Number of deaths / Total cases) × 100
- (Number of cases / Number of susceptible individuals) × 100
- (Number of exposures / Total population) × 100
Correct answer: (Number of new cases / Total population at risk) × 100
The attack rate equals the number of new cases divided by the total population at risk during a specified time period, expressed as a percentage.
Question 6: A researcher enrolls 500 healthy adults and follows them for 10 years to track development of cardiovascular disease. This study design is:
- Case-control study
- Cross-sectional study
- Prospective cohort study (Correct answer)
- Retrospective cohort study
Correct answer: Prospective cohort study
Enrolling disease-free participants at baseline and following them forward in time to observe new outcomes defines a prospective cohort study.
Question 7: Confounding in an epidemiological study occurs when:
- Exposure and outcome are measured at the same point in time
- A third variable is associated with both the exposure and the outcome but is not on the causal pathway (Correct answer)
- Participants are lost to follow-up at differential rates
- Measurement instruments introduce systematic error
Correct answer: A third variable is associated with both the exposure and the outcome but is not on the causal pathway
A confounder must be associated with the exposure, associated with the outcome, and not be an intermediate step between them; it creates a spurious or distorted association.
NBME Biostatistics and Epidemiology 2
Which measure of association is used in a case-control study?