BMAT Research & Evidence-Based Practice 2 — Questions and Answers
Question 1: A researcher wants to determine whether a new drug reduces blood pressure. Which study design provides the strongest causal evidence?
- Cross-sectional survey
- Case-control study
- Randomized controlled trial (Correct answer)
- Ecological study
Correct answer: Randomized controlled trial
RCTs use randomization to control confounding, making them the gold standard for establishing causality.
Question 2: In a clinical trial, 'allocation concealment' is used primarily to prevent:
- Participants dropping out of the study
- Selection bias during group assignment (Correct answer)
- Measurement error in outcomes
- Loss to follow-up
Correct answer: Selection bias during group assignment
Allocation concealment prevents those enrolling participants from knowing the upcoming assignment, preventing selection bias.
Question 3: A study reports a hazard ratio of 0.6 (95% CI: 0.4–0.9) for a new treatment vs. placebo. Which interpretation is correct?
- The treatment increases the hazard by 40%
- The result is not statistically significant
- The treatment reduces the hazard by 40% (Correct answer)
- The confidence interval indicates no effect
Correct answer: The treatment reduces the hazard by 40%
A hazard ratio of 0.6 means the treatment group has a 40% lower hazard rate; the CI excludes 1.0 so the result is significant.
Question 4: Which of the following best describes 'publication bias' in the context of systematic reviews?
- Reviewers preferring studies from high-impact journals
- Studies with positive results being more likely to be published (Correct answer)
- Researchers selectively reporting outcomes
- Duplicate publication of the same dataset
Correct answer: Studies with positive results being more likely to be published
Publication bias occurs when studies with statistically significant (positive) results are more likely to be published, skewing meta-analyses.
Question 5: A screening test for cancer has sensitivity 90% and specificity 80%. In a population with 1% disease prevalence, what is the approximate positive predictive value?
- 90%
- 80%
- 4% (Correct answer)
- 50%
Correct answer: 4%
With low prevalence, most positive tests are false positives; applying Bayes' theorem gives a PPV of approximately 4.3%.
Question 6: When appraising a cohort study, which factor most threatens internal validity?
- Large sample size
- Differential loss to follow-up between exposed and unexposed groups (Correct answer)
- Long follow-up duration
- Blinding of outcome assessors
Correct answer: Differential loss to follow-up between exposed and unexposed groups
If participants drop out differently by exposure group, the results may be biased rather than reflecting the true exposure-outcome relationship.
Question 7: The Number Needed to Treat (NNT) is calculated as:
- 1 divided by the relative risk
- 1 divided by the absolute risk reduction (Correct answer)
- Relative risk reduction divided by baseline risk
- Odds ratio minus 1
Correct answer: 1 divided by the absolute risk reduction
NNT = 1 / ARR, where ARR is the absolute difference in event rates between control and treatment groups.
BMAT Research & Evidence-Based Practice 2
A researcher wants to determine whether a new drug reduces blood pressure.
Which study design provides the strongest causal evidence?