BHS Epidemiology 4 — Questions and Answers
Question 1: Lead-time bias in cancer screening studies causes an apparent improvement in survival because:
- Screening selects healthier patients
- Survival is measured from diagnosis, which is advanced earlier (Correct answer)
- Screened patients receive better treatment
- Mild cases are excluded from analysis
Correct answer: Survival is measured from diagnosis, which is advanced earlier
Lead-time bias inflates survival time by starting the clock at an earlier diagnosis without the person actually living longer.
Question 2: Herd immunity is achieved when the proportion of immune individuals in a population reaches:
- 50%
- 1 − (1/R₀) (Correct answer)
- R₀ / (R₀ − 1)
- The attack rate threshold
Correct answer: 1 − (1/R₀)
The herd immunity threshold is 1 − 1/R₀; above this fraction, each case on average produces fewer than one new case and the epidemic declines.
Question 3: In a 2×2 contingency table for a case-control study, the measure of association calculated is the:
- Risk ratio
- Incidence rate ratio
- Odds ratio (Correct answer)
- Prevalence ratio
Correct answer: Odds ratio
Because case-control studies do not follow a population over time, incidence cannot be calculated directly; the odds ratio approximates relative risk.
Question 4: Which of the following is a strength of ecological studies compared to individual-level studies?
- Ability to draw individual-level causal inferences
- Can utilize routinely collected population-level data at low cost (Correct answer)
- Eliminates confounding
- Provides precise exposure measurements
Correct answer: Can utilize routinely collected population-level data at low cost
Ecological studies are efficient and inexpensive because they use existing aggregate data such as disease registries and census records.
Question 5: A study examining disease rates in populations with different fluoride levels in water is an example of:
- Nested case-control study
- Ecological study (Correct answer)
- Cross-sectional survey
- Natural experiment
Correct answer: Ecological study
Ecological studies compare disease rates across populations defined by average exposure levels rather than measuring individual exposures.
Question 6: Information bias due to differential misclassification of exposure between cases and controls is called:
- Recall bias (Correct answer)
- Length-biased sampling
- Confounding
- Attrition bias
Correct answer: Recall bias
Recall bias occurs when cases remember past exposures more (or less) accurately than controls, distorting the association.
Question 7: Which surveillance system characteristic ensures that cases from a defined population are consistently reported over time?
- Representativeness (Correct answer)
- Completeness
- Timeliness
- Simplicity
Correct answer: Representativeness
Representativeness ensures that detected cases accurately reflect the distribution of disease in the population across time, place, and person.
BHS Epidemiology 4
Lead-time bias in cancer screening studies causes an apparent improvement in survival because: