Odds ratio vs. relative risk — MCQs

A research fellow proposes a nested case-control study within an existing cohort examining antibiotic exposure and C. difficile infection. The mentor suggests this design wastes the cohort structure and that relative risk should be calculated instead. The fellow argues that odds ratios from nested case-control studies approximate relative risk while being more efficient. Evaluate the validity of each position and synthesize the optimal approach.

Two studies examine statin therapy and stroke prevention. Study A (cohort, n=10,000) reports RR=0.75. Study B (case-control, n=2,000) reports OR=0.68. The absolute stroke rate in the general population is 2% over 5 years. Analyze these findings to determine which study provides more accurate information for clinical decision-making and why.

A genetic epidemiology study uses a case-control design to examine BRCA1 mutations and breast cancer risk, reporting an odds ratio of 15.0 (95% CI: 8.2-27.4). A patient with a BRCA1 mutation asks what this means for her actual risk of developing breast cancer. Evaluate how to appropriately counsel this patient regarding the study findings.

A hospital quality improvement team analyzes surgical site infections (SSI) following colorectal surgery. In Year 1 (pre-intervention), 50 of 500 surgeries resulted in SSI. After implementing a bundle protocol in Year 2, 20 of 500 surgeries resulted in SSI. The team debates whether to report odds ratio or relative risk. Apply your understanding to select the most appropriate measure and calculate it.

A cross-sectional study of 5,000 adults examines the association between obesity (BMI ≥30) and diabetes. Results show 800 obese individuals with 240 having diabetes, and 4,200 non-obese individuals with 420 having diabetes. The researchers calculate an odds ratio of 3.2. A reviewer criticizes the choice of odds ratio for a cross-sectional design. Evaluate which measure is most appropriate and why.

A meta-analysis combines data from 5 case-control studies and 3 cohort studies examining the association between NSAID use and myocardial infarction. The case-control studies report odds ratios ranging from 1.3-1.8, while cohort studies report relative risks of 1.2-1.4. Analyze why these measures differ and evaluate the appropriate approach for combining these studies.

A pharmaceutical company conducts a randomized controlled trial of a new anticoagulant. After analyzing 2,000 patients (1,000 per arm), they report an odds ratio of 0.6 for stroke prevention compared to standard therapy. The medical director questions why odds ratio was used instead of relative risk for a prospective randomized trial. Apply your understanding to determine the most appropriate measure and rationale.

A retrospective case-control study examines the association between oral contraceptive use and venous thromboembolism (VTE). The calculated odds ratio is 3.2 with a 95% confidence interval of 2.1-4.8. A colleague suggests converting this to relative risk to better communicate the findings to patients. Evaluate the appropriateness of this conversion.

A cohort study follows 1,000 healthcare workers for 5 years to assess the relationship between annual influenza vaccination and development of influenza infection. Among 600 vaccinated workers, 30 developed influenza. Among 400 unvaccinated workers, 80 developed influenza. Calculate the relative risk of developing influenza for vaccinated versus unvaccinated workers.

A research team conducts a case-control study examining the relationship between smoking and lung cancer. They identify 200 patients with lung cancer (cases) and 200 matched controls without lung cancer. Among cases, 160 were smokers and 40 were non-smokers. Among controls, 80 were smokers and 120 were non-smokers. Calculate the odds ratio for this study.