Questions
A clinical trial compares two chemotherapy regimens for lung cancer. The trial reports a p-value of 0.03 for the difference in median survival times. What does this p-value indicate?
a) There is a 3% chance that the null hypothesis is true.
b) There is a 97% chance that the alternative hypothesis is true.
c) If the null hypothesis is true, there is a 3% probability of observing a difference at least as extreme as the one observed.
d) The difference in survival times is clinically significant.
e) The trial has a 3% chance of being underpowered.In a case-control study investigating smoking and bladder cancer, the odds ratio is 2.5 (95% CI: 1.8–3.4). What is the most appropriate interpretation of this odds ratio?
a) Smokers are 2.5 times more likely to develop bladder cancer than non-smokers.
b) The risk of bladder cancer is 2.5 times higher in smokers compared to non-smokers.
c) The odds of smoking are 2.5 times higher in bladder cancer cases than in controls.
d) Smoking causes a 2.5-fold increase in bladder cancer incidence.
e) The probability of bladder cancer is 2.5 times higher in smokers.A randomized controlled trial (RCT) uses a two-sample t-test to compare mean pain scores between two groups. What additional assumption does the two-sample t-test make compared to a paired t-test?
a) The data must be normally distributed.
b) The variances of the two groups must be equal.
c) The observations must be paired.
d) The sample sizes must be equal.
e) The data must be categorical.In a study of breast cancer screening, the sensitivity of a mammogram is 90%, and the specificity is 95%. If the prevalence of breast cancer in the screened population is 2%, what is the positive predictive value (PPV)?
a) 27.3%
b) 90.0%
c) 95.0%
d) 98.0%
e) 99.0%A survival analysis compares two cancer treatments using a log-rank test. The test yields a p-value of 0.08. What is the most appropriate conclusion at a 5% significance level?
a) The treatments have significantly different survival distributions.
b) There is insufficient evidence to reject the null hypothesis of equal survival distributions.
c) The test is underpowered to detect a difference.
d) The hazard ratio is equal to 1.
e) The survival curves are identical.A cohort study reports a relative risk of 1.8 for lung cancer in asbestos-exposed workers compared to unexposed workers. The 95% confidence interval is 1.2–2.7. What does the confidence interval indicate?
a) The true relative risk is exactly 1.8.
b) There is a 95% chance the relative risk is between 1.2 and 2.7.
c) If the study is repeated, 95% of confidence intervals will contain the true relative risk.
d) The result is not statistically significant.
e) The relative risk is biased.In a clinical trial, the number needed to treat (NNT) for a new drug to prevent one additional cancer recurrence is 20. What does this NNT mean?
a) 20% of patients will benefit from the drug.
b) 20 patients must be treated to prevent one recurrence.
c) The drug reduces recurrence risk by 20%.
d) The absolute risk reduction is 20%.
e) The drug is effective in 1 out of 20 patients.A study uses a chi-square test to assess the association between radiation exposure and thyroid cancer. The test statistic is 5.99 with 1 degree of freedom, and the p-value is 0.014. What is the correct interpretation?
a) There is no association between radiation and thyroid cancer.
b) The association is statistically significant at the 5% level.
c) The test is invalid due to low degrees of freedom.
d) The odds ratio is 5.99.
e) The result is clinically insignificant.In a meta-analysis of cancer therapies, a forest plot shows a pooled hazard ratio of 0.75 (95% CI: 0.60–0.90). What is the most appropriate interpretation?
a) The therapy increases the risk of death by 25%.
b) The therapy reduces the hazard of death by 25%.
c) The therapy has no effect on survival.
d) The confidence interval indicates a non-significant result.
e) The hazard ratio is biased due to heterogeneity.A study reports a standardized incidence ratio (SIR) of 1.5 for leukemia in a population exposed to radiation. What does the SIR represent?
a) The relative risk of leukemia compared to a control group.
b) The ratio of observed to expected leukemia cases in the exposed population.
c) The absolute risk of leukemia in the exposed population.
d) The odds of leukemia in the exposed versus unexposed population.
e) The proportion of leukemia cases attributable to radiation.
Answers
Answer: c) If the null hypothesis is true, there is a 3% probability of observing a difference at least as extreme as the one observed.
Explanation: The p-value represents the probability of obtaining a result at least as extreme as the observed one, assuming the null hypothesis is true. It does not indicate the probability of hypotheses being true or clinical significance.Answer: c) The odds of smoking are 2.5 times higher in bladder cancer cases than in controls.
Explanation: In a case-control study, the odds ratio compares the odds of exposure (smoking) in cases versus controls. It does not directly measure risk or probability, as these require a cohort study.Answer: b) The variances of the two groups must be equal.
Explanation: The two-sample t-test assumes equal variances between groups (often tested with Levene’s statistic), while the paired t-test accounts for paired data and does not require this assumption. Normality is assumed for both.Answer: a) 27.3%
Explanation: PPV = (Sensitivity × Prevalence) / [(Sensitivity × Prevalence) + (1 – Specificity) × (1 – Prevalence)]. Using Sensitivity = 0.9, Specificity = 0.95, Prevalence = 0.02: PPV = (0.9 × 0.02) / [(0.9 × 0.02) + (1 – 0.95) × (1 – 0.02)] = 0.018 / (0.018 + 0.049) ≈ 0.273 or 27.3%.Answer: b) There is insufficient evidence to reject the null hypothesis of equal survival distributions.
Explanation: At a 5% significance level (p < 0.05), a p-value of 0.08 is not significant, so we fail to reject the null hypothesis. The p-value does not directly address power or hazard ratios.Answer: c) If the study is repeated, 95% of confidence intervals will contain the true relative risk.
Explanation: A 95% confidence interval means that if the study is repeated many times, 95% of the calculated intervals will contain the true relative risk. It does not give the probability of the true value or indicate bias.Answer: b) 20 patients must be treated to prevent one recurrence.
Explanation: NNT is the number of patients who need to be treated to achieve one additional beneficial outcome (e.g., prevent one recurrence). It is calculated as 1 / Absolute Risk Reduction.Answer: b) The association is statistically significant at the 5% level.
Explanation: A p-value of 0.014 is less than 0.05, indicating a statistically significant association at the 5% level. The chi-square test statistic and degrees of freedom are valid, and clinical significance is not addressed by p-values.Answer: b) The therapy reduces the hazard of death by 25%.
Explanation: A hazard ratio of 0.75 indicates a 25% reduction in the hazard of death (1 – 0.75 = 0.25). The 95% CI (0.60–0.90) does not include 1, confirming significance.Answer: b) The ratio of observed to expected leukemia cases in the exposed population.
Explanation: The SIR compares the observed number of cases in a population to the expected number based on a reference population, often used in cancer epidemiology. It is not a relative risk or odds ratio.
