Concepts of risk: attributable risk, relative risk, odds ratio MCQs With Answer

Introduction

Concepts of risk — attributable risk, relative risk, and odds ratio — are foundational to pharmacoepidemiology and pharmacoeconomics. This quiz collection clarifies definitions, formulas, interpretation, and application of these measures in cohort and case–control studies. M. Pharm students will practice calculations (risk difference, risk ratio, attributable fraction, population attributable risk, and odds ratio), distinguish when each measure is appropriate, and learn how study design, disease prevalence, and confounding affect estimates. Detailed MCQs emphasize clinical interpretation and public‑health implications, helping you evaluate causality, quantify preventable cases, and choose correct analytic approaches for pharmacoepidemiologic research.

Q1. Which of the following best defines attributable risk (risk difference)?

• The proportion of all cases in the population attributable to the exposure
• The ratio of incidence in exposed to incidence in unexposed
• The difference between incidence in exposed and incidence in unexposed (Ie − Iu)
• The odds of exposure among cases divided by odds among controls

Correct Answer: The difference between incidence in exposed and incidence in unexposed (Ie − Iu)

Q2. Which formula correctly expresses relative risk (RR)?

• RR = (odds of exposure among cases) / (odds of exposure among controls)
• RR = (incidence in exposed) / (incidence in unexposed)
• RR = (incidence in unexposed) − (incidence in exposed)
• RR = (prevalence in exposed) / (prevalence in unexposed)

Correct Answer: RR = (incidence in exposed) / (incidence in unexposed)

Q3. In a case–control study which statement about the odds ratio (OR) is correct?

• OR directly measures incidence rates and requires cohort data
• OR estimates the relative risk and approximates RR when the disease is rare
• OR equals attributable risk when exposure is common
• OR is only valid for matched cohort studies

Correct Answer: OR estimates the relative risk and approximates RR when the disease is rare

Q4. Which pair correctly matches study design to the typical effect measure reported?

• Cross‑sectional: Relative Risk; Cohort: Odds Ratio
• Cohort: Relative Risk; Case–control: Odds Ratio
• Case–control: Relative Risk; Randomized trial: Odds Ratio
• Cohort: Odds Ratio; Case–control: Attributable risk

Correct Answer: Cohort: Relative Risk; Case–control: Odds Ratio

Q5. In a cohort study incidence in exposed = 0.20 and incidence in unexposed = 0.05. What is the attributable risk (risk difference)?

• 0.25 (25%)
• 0.15 (15%)
• 4.0 (400%)
• 0.05 (5%)

Correct Answer: 0.15 (15%)

Q6. Using the same incidences (Ie = 0.20, Iu = 0.05), what is the attributable fraction among the exposed (AFExposed)?

• 25%
• 75%
• 15%
• 5%

Correct Answer: 75%

Q7. Which formula gives the population attributable fraction (PAR%) when Pe is prevalence of exposure and RR is relative risk?

• PAR% = [Pe × (RR − 1)] / [Pe × (RR − 1) + 1] × 100
• PAR% = (RR − 1) / RR × 100
• PAR% = (Ie − Iu) / Ie × 100
• PAR% = (odds exposed among cases − odds exposed among controls) × 100

Correct Answer: PAR% = [Pe × (RR − 1)] / [Pe × (RR − 1) + 1] × 100

Q8. From a case–control study: cases exposed = 40, cases unexposed = 60; controls exposed = 20, controls unexposed = 80. What is the odds ratio?

• 0.75
• 1.33
• 2.67
• 4.00

Correct Answer: 2.67

Q9. If a cohort study reports RR = 0.6 for a drug exposure and adverse outcome, how should this be interpreted?

• The exposure increases risk by 60%
• The exposure has no effect on risk
• The exposure reduces the risk by 40%
• The exposure causes the outcome in 60% of exposed

Correct Answer: The exposure reduces the risk by 40%

Q10. When disease is rare, which statement is true about OR and RR?

• OR substantially underestimates RR
• OR approximates RR and can be interpreted similarly for strength of association
• OR cannot be calculated from case–control studies
• RR is always larger than OR

Correct Answer: OR approximates RR and can be interpreted similarly for strength of association

Q11. Which measure is independent of disease prevalence and can be calculated in case–control studies without incidence data?

• Relative risk (RR)
• Attributable risk (risk difference)
• Odds ratio (OR)
• Incidence rate ratio (IRR)

Correct Answer: Odds ratio (OR)

Q12. For a matched case–control study analysed in pairs, which test is appropriate to assess association?

• Unpaired Chi‑square test
• Paired t‑test
• McNemar’s test
• ANOVA

Correct Answer: McNemar’s test

Q13. If RR = 2.0 and incidence in unexposed (Iu) = 0.10, what is the incidence in exposed (Ie)?

• 0.05
• 0.10
• 0.20
• 0.30

Correct Answer: 0.20

Q14. If prevalence of exposure in population Pe = 0.30 and RR = 3.0, what is the population attributable fraction (PAR%) approximately?

• 18.8%
• 37.5%
• 60.0%
• 90.0%

Correct Answer: 37.5%

Q15. Which method provides a pooled adjusted estimate of association across strata controlling for one categorical confounder?

• Crude odds ratio only
• Mantel–Haenszel (pooled) odds ratio
• Simple difference of means
• Kaplan–Meier estimator

Correct Answer: Mantel–Haenszel (pooled) odds ratio

Q16. True or false: For common outcomes, the odds ratio often overestimates the relative risk.

• True
• False
• Only true for protective exposures
• Only true in randomized trials

Correct Answer: True

Q17. If a study finds RR = 1.0 for exposure and outcome, what is the attributable fraction among exposed (AFExposed)?

• 100%
• 50%
• 0%
• Cannot be determined

Correct Answer: 0%

Q18. Under which condition do the odds ratio and relative risk approximate each other most closely?

• When outcome incidence exceeds 50%
• When the outcome is rare (e.g., incidence <10%)
• When exposure prevalence is 100%
• When sample sizes are extremely large regardless of incidence

Correct Answer: When the outcome is rare (e.g., incidence <10%)

Q19. An attributable risk of 0.12 in a cohort study means:

• 12 fewer cases per 100 exposed compared with unexposed
• 12 additional cases per 100 exposed compared with unexposed
• Exposed individuals are 12 times more likely to develop the outcome
• 12% of the population is exposed

Correct Answer: 12 additional cases per 100 exposed compared with unexposed

Q20. Which formula gives the odds ratio expressed using incidences in a cohort (Ie and Iu)?

• OR = (Ie − Iu)
• OR = (Ie / (1 − Ie)) / (Iu / (1 − Iu))
• OR = Ie / Iu
• OR = (Ie − Iu) / Iu

Correct Answer: OR = (Ie / (1 − Ie)) / (Iu / (1 − Iu))

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