Cohort studies and incidence/prevalence measures MCQs With Answer

Cohort studies and incidence/prevalence measures MCQs With Answer

This quiz collection is designed for M.Pharm students to deepen understanding of cohort study designs and core epidemiological measures such as incidence, prevalence, person-time, risk ratios and attributable measures. Each question targets practical and conceptual aspects encountered in pharmacoepidemiology — including prospective versus retrospective cohorts, calculation of incidence and prevalence, interpretation of rates and risks, biases specific to cohort designs, and analytic methods for controlling confounding. The questions range from foundational definitions to applied problems requiring calculation or critical appraisal, with answers provided for self-assessment and exam preparation.

Q1. Which statement best describes a prospective cohort study?

• A study that enrolls participants based on disease status and looks backward for exposure
• A cross-sectional assessment of exposure and outcome at a single time point
• A study that enrolls participants free of outcome and follows exposed and unexposed forward to observe incident outcomes
• A randomized assignment of exposure to measure causal effect

Correct Answer: A study that enrolls participants free of outcome and follows exposed and unexposed forward to observe incident outcomes

Q2. Which pair correctly contrasts incidence and prevalence?

• Incidence measures existing cases at a time; prevalence measures new cases over time
• Incidence measures new cases over a specified time; prevalence measures all existing cases at a point or period
• Both incidence and prevalence measure only chronic diseases
• Prevalence is always lower than incidence for any disease

Correct Answer: Incidence measures new cases over a specified time; prevalence measures all existing cases at a point or period

Q3. What is the main conceptual difference between cumulative incidence and incidence rate (incidence density)?

• Cumulative incidence uses person-time in the denominator; incidence rate uses only the initial population size
• Cumulative incidence is a proportion of new cases over a defined period; incidence rate accounts for person-time at risk and can handle variable follow-up
• They are identical measures and always yield the same numeric value
• Cumulative incidence is used for rare diseases only; incidence rate is for common diseases

Correct Answer: Cumulative incidence is a proportion of new cases over a defined period; incidence rate accounts for person-time at risk and can handle variable follow-up

Q4. Which formula correctly represents the incidence rate (incidence density)?

• Number of new cases during period / Population at start of period
• Number of existing cases at a point in time / Population at the same time
• Number of new cases during period / Total person-time at risk during period
• Number of deaths during period / Number of cases during period

Correct Answer: Number of new cases during period / Total person-time at risk during period

Q5. How is point prevalence defined?

• Number of new cases over one year / average population during that year
• Number of existing cases at a specific point in time / population at that same point in time
• Number of exposures in a cohort / total person-time
• Number of deaths due to disease / total number of cases

Correct Answer: Number of existing cases at a specific point in time / population at that same point in time

Q6. Under steady-state conditions (stable incidence and duration), which approximation relating prevalence (P), incidence (I) and average duration (D) is generally used?

• P ≈ I / D
• P ≈ I × D
• P ≈ I + D
• P ≈ I − D

Correct Answer: P ≈ I × D

Q7. What does the risk ratio (relative risk) compare in a cohort study?

• Incidence in the unexposed divided by incidence in the exposed
• Prevalence in the control group divided by prevalence in the treatment group
• Incidence in the exposed divided by incidence in the unexposed
• Odds of exposure among cases divided by odds of exposure among controls

Correct Answer: Incidence in the exposed divided by incidence in the unexposed

Q8. Which measure represents the absolute excess risk of disease attributable to exposure?

• Risk ratio (relative risk)
• Attributable risk (risk difference)
• Odds ratio
• Population attributable fraction

Correct Answer: Attributable risk (risk difference)

Q9. Which formula is the correct expression for the population attributable fraction (PAF) when prevalence of exposure among the population is Pe and relative risk is RR?

• PAF = (RR − 1) / RR
• PAF = Pe × (RR − 1) / [Pe × (RR − 1) + 1]
• PAF = RR / Pe
• PAF = (Incidence in exposed − Incidence in unexposed) / Incidence in exposed

Correct Answer: PAF = Pe × (RR − 1) / [Pe × (RR − 1) + 1]

Q10. Three participants contribute follow-up of 5, 3 and 2 years before outcome or censoring. What is the total person-years at risk?

• 3 person-years
• 5 person-years
• 10 person-years
• 15 person-years

Correct Answer: 10 person-years

Q11. Which statement correctly characterizes incidence density compared to cumulative incidence for rare events?

• Incidence density cannot be used for rare events
• For rare events and short follow-up, incidence density approximates cumulative incidence
• Cumulative incidence always exceeds incidence density numerically
• They measure different constructs and are never comparable

Correct Answer: For rare events and short follow-up, incidence density approximates cumulative incidence

Q12. Loss to follow-up in cohort studies most directly threatens which of the following?

• Randomization integrity
• External validity only, with no impact on internal validity
• Internal validity due to potential selection bias if loss is related to both exposure and outcome
• Laboratory assay precision

Correct Answer: Internal validity due to potential selection bias if loss is related to both exposure and outcome

Q13. What is “immortal time bias” in observational cohort studies?

• A bias from misclassification of exposure that is unrelated to follow-up time
• A period of follow-up during which, by design, the outcome cannot occur for subjects classified in the exposed group, leading to biased effect estimates
• A temporary increase in incidence due to seasonal factors
• A measurement error in outcome assessment that biases toward the null

Correct Answer: A period of follow-up during which, by design, the outcome cannot occur for subjects classified in the exposed group, leading to biased effect estimates

Q14. Which feature distinguishes a nested case-control study from a case-cohort study?

• Nested case-control selects controls matched to each case at risk time; case-cohort samples a subcohort at baseline usable as controls for multiple cases
• Nested case-control is prospective while case-cohort is always retrospective
• Case-cohort requires randomization; nested case-control does not
• There is no difference; they are identical designs

Correct Answer: Nested case-control selects controls matched to each case at risk time; case-cohort samples a subcohort at baseline usable as controls for multiple cases

Q15. Which best defines period prevalence?

• Number of individuals who ever had the disease during a specified period divided by the average population during that period
• Number of new cases during a period divided by the population at risk at the start
• Number of current cases at a single day divided by the population that day
• Number of deaths during a period divided by the number of cases

Correct Answer: Number of individuals who ever had the disease during a specified period divided by the average population during that period

Q16. Which type of study is least susceptible to reverse causation?

• Cross-sectional study
• Case-control study
• Prospective cohort study
• Ecological study

Correct Answer: Prospective cohort study

Q17. How is a hazard ratio (from a Cox proportional hazards model) interpreted compared with a risk ratio?

• Hazard ratio compares instantaneous event rates over time and can approximate a risk ratio when hazards are proportional and follow-up is short
• Hazard ratio is identical to odds ratio and unrelated to time-to-event
• Hazard ratio is used only for prevalence data
• Hazard ratio is always smaller than the corresponding risk ratio

Correct Answer: Hazard ratio compares instantaneous event rates over time and can approximate a risk ratio when hazards are proportional and follow-up is short

Q18. Which analytic method is commonly used in cohort studies to control confounding by measured variables?

• Random digit dialing
• Stratification and multivariable regression (e.g., Cox or Poisson regression)
• Blinding participants to exposure
• Increasing loss to follow-up

Correct Answer: Stratification and multivariable regression (e.g., Cox or Poisson regression)

Q19. Non-differential misclassification of a binary exposure in a cohort study typically biases the estimated association in which direction?

• Toward the null (attenuation of effect)
• Away from the null (exaggeration of effect)
• Always creates a positive association regardless of truth
• Has no predictable effect on bias

Correct Answer: Toward the null (attenuation of effect)

Q20. Which factor will generally increase the statistical power of a cohort study to detect a given effect size?

• Lower incidence of the outcome in both exposure groups
• Shorter duration of follow-up
• Increasing total person-time (longer follow-up or larger sample size)
• Increasing measurement error in exposure assessment

Correct Answer: Increasing total person-time (longer follow-up or larger sample size)

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