OCC OCC Evidence-Based Medicine & Research 2 — Questions and Answers
Question 1: Which bias occurs when patients who are lost to follow-up in a study differ systematically from those who complete the study?
- Attrition bias (loss to follow-up bias) (Correct answer)
- Selection bias
- Recall bias
- Detection bias
Correct answer: Attrition bias (loss to follow-up bias)
Attrition bias occurs when participants who withdraw or are lost differ from those who remain, potentially skewing results.
Question 2: In a cohort study examining smoking and lung cancer, researchers find a hazard ratio (HR) of 3.2. What does this represent?
- Smokers are 3.2 times more likely to develop lung cancer over time than non-smokers (Correct answer)
- 32% of smokers develop lung cancer
- Non-smokers have 3.2x more risk than smokers
- 3.2 additional cancers per 100 smokers per year
Correct answer: Smokers are 3.2 times more likely to develop lung cancer over time than non-smokers
A hazard ratio of 3.2 means the exposed group (smokers) have 3.2 times the instantaneous rate of the outcome (lung cancer) compared to the unexposed.
Question 3: A p-value of 0.03 is reported in a clinical trial. What does this indicate?
- There is a 3% probability that the observed difference occurred by chance if the null hypothesis is true (Correct answer)
- The treatment is 97% effective
- The study has 97% power
- There is a 97% chance the results are clinically meaningful
Correct answer: There is a 3% probability that the observed difference occurred by chance if the null hypothesis is true
A p-value is the probability of observing results as extreme as those found, assuming the null hypothesis (no difference) is true.
Question 4: Which study design is most appropriate for investigating the cause of a rare disease?
- Case-control study (Correct answer)
- Randomized controlled trial
- Prospective cohort study
- Cross-sectional study
Correct answer: Case-control study
Case-control studies are efficient for rare diseases because they start with cases (people who have the disease) and work backward to identify exposures.
Question 5: What is the purpose of an intention-to-treat (ITT) analysis in a randomized controlled trial?
- Analyze all randomized participants in their originally assigned groups regardless of protocol adherence (Correct answer)
- Only analyze participants who completed the full treatment protocol
- Exclude participants who had adverse events
- Compare outcomes only in those who responded to treatment
Correct answer: Analyze all randomized participants in their originally assigned groups regardless of protocol adherence
ITT analysis preserves randomization and prevents overestimation of efficacy by including all participants in their originally assigned groups.
Question 6: Confidence intervals (CI) that cross 1.0 for relative risk or odds ratio indicate what about statistical significance?
- The result is not statistically significant at the chosen alpha level (Correct answer)
- The result is highly significant
- The sample size is adequate
- The study has low risk of bias
Correct answer: The result is not statistically significant at the chosen alpha level
When a 95% CI for RR or OR crosses 1.0, it includes the null hypothesis of no effect, meaning the result is not statistically significant at p < 0.05.
OCC OCC Evidence-Based Medicine & Research 2
Which bias occurs when patients who are lost to follow-up in a study differ systematically from those who complete the study?