RCTs Practice Questions

Q: A randomized controlled trial is conducted investigating the effects of different diagnostic imaging modalities on breast cancer mortality. 8,000 women are randomized to receive either conventional mammography or conventional mammography with breast MRI. The primary outcome is survival from the time of breast cancer diagnosis. The conventional mammography group has a median survival after diagnosis of 17.0 years. The MRI plus conventional mammography group has a median survival of 19.5 years. If this difference is statistically significant, which form of bias may be affecting the results?

Lead-time bias. ***Lead-time bias*** - This bias occurs when a screening test diagnoses a disease earlier, making **survival appear longer** even if the actual time of death is unchanged. - In this scenario, adding **MRI** may detect breast cancer at an earlier, asymptomatic stage, artificially extending the apparent survival duration from diagnosis without necessarily changing the ultimate prognosis. *Recall bias* - **Recall bias** applies to retrospective studies where subjects are asked to recall past exposures, and those with the outcome are more likely to remember potential exposures. - It's irrelevant here as this is a **prospective randomized controlled trial** studying objective survival outcomes, not subjective past recollections. *Selection bias* - **Selection bias** occurs when participants are not randomly assigned to groups, leading to systematic differences between the groups influencing the outcome. - This study is a **randomized controlled trial**, which is designed to minimize selection bias by ensuring participants have an equal chance of being assigned to either treatment arm. *Misclassification bias* - **Misclassification bias** happens when either the exposure or the outcome is incorrectly categorized, leading to erroneous associations. - This study uses objective diagnostic imaging and survival data, thus reducing the likelihood of **misclassification of diagnosis or survival status**. *Because this study is a randomized controlled trial, it is free of bias* - While **randomized controlled trials (RCTs)** are considered the **gold standard** for minimizing bias, they are not entirely immune to all forms of bias. - **Lead-time bias**, for instance, can still occur in RCTs involving screening or early diagnosis, as seen in this example, and other biases like **information bias** or **reporting bias** can also arise.

Q: A group of bariatric surgeons are investigating a novel surgically-placed tube that drains a portion of the stomach following each meal. They are interested in studying its efficacy in facilitating weight loss in obese adults with BMIs > 40 kg/m2 who have failed to lose weight through non-surgical options. After randomizing 150 patients to undergoing the surgical tube procedure and 150 patients to non-surgical weight loss options (e.g., diet, exercise), the surgeons found that, on average, participants in the surgical treatment group lost 15% of their total body weight in comparison to 4% in the non-surgical group. Which of the following statistical tests is an appropriate initial test to evaluate if this difference in weight loss between the two groups is statistically significant?

Unpaired two-sample t-test. ***Unpaired two-sample t-test*** - The goal is to compare the **means of two independent groups** (surgical vs. non-surgical) on a continuous outcome (percentage of weight loss). - An unpaired t-test is ideal for determining if the **observed difference between these two group means** is statistically significant. *Kaplan-Meier analysis* - This analysis is used to estimate and compare **survival curves** or time-to-event data between groups. - It is not suitable for comparing the **mean weight loss** between two independent groups. *Paired two-sample t-test* - A paired t-test is used when comparing two measurements from the **same individuals** or **matched pairs**. - Here, the two groups are distinct and independent, not paired in any way. *Multiple linear regression* - This is used to model the **relationship between a dependent variable** and **two or more independent variables**. - While useful for predicting weight loss based on multiple factors, it's not the most direct or initial test for simply comparing the mean weight loss between two groups. *Pearson correlation coefficient* - The Pearson correlation coefficient measures the **strength and direction of a linear relationship between two continuous variables**. - It does not compare the means of two independent groups, but rather assesses the **degree to which two variables change together**.

Question 1

A study seeks to investigate the therapeutic efficacy of treating asymptomatic subclinical hypothyroidism in preventing symptoms of hypothyroidism. The investigators found 300 asymptomatic patients with subclinical hypothyroidism, defined as serum thyroid-stimulating hormone (TSH) of 5 to 10 μU/mL with normal serum thyroxine (T4) levels. The patients were randomized to either thyroxine 75 μg daily or placebo. Both investigators and study subjects were blinded. Baseline patient characteristics were distributed similarly in the treatment and control group (p > 0.05). Participants' serum T4 and TSH levels and subjective quality of life were evaluated at a 3-week follow-up. No difference was found between the treatment and placebo groups. Which of the following is the most likely explanation for the results of this study?

Accepted Answer

Latency period

Answer

Observer effect

Answer

Berkson bias

Answer

Confounding bias

Answer

Lead-time bias

Question 2

In the study, all participants who were enrolled and randomly assigned to treatment with pulmharkimab were analyzed in the pulmharkimab group regardless of medication nonadherence or refusal of allocated treatment. A medical student reading the abstract is confused about why some participants assigned to pulmharkimab who did not adhere to the regimen were still analyzed as part of the pulmharkimab group. Which of the following best reflects the purpose of such an analysis strategy?

Accepted Answer

To reduce selection bias

Answer

To minimize type 2 errors

Answer

To assess treatment efficacy more accurately

Answer

To increase internal validity of study

Answer

To increase sample size

Question 3

A randomized control double-blind study is conducted on the efficacy of 2 sulfonylureas. The study concluded that medication 1 was more efficacious in lowering fasting blood glucose than medication 2 (p ≤ 0.05; 95% CI: 14 [10-21]). Which of the following is true regarding a 95% confidence interval (CI)?

Accepted Answer

If the same study were repeated multiple times, approximately 95% of the calculated confidence intervals would contain the true population parameter.

Answer

The 95% confidence interval is the probability chosen by the researcher to be the threshold of statistical significance.

Answer

When a 95% CI for the estimated difference between groups contains the value ‘0’, the results are significant.

Answer

It represents the probability that chance would not produce the difference shown, 95% of the time.

Answer

The study is adequately powered at the 95% confidence interval.

Question 4

A randomized controlled trial is conducted investigating the effects of different diagnostic imaging modalities on breast cancer mortality. 8,000 women are randomized to receive either conventional mammography or conventional mammography with breast MRI. The primary outcome is survival from the time of breast cancer diagnosis. The conventional mammography group has a median survival after diagnosis of 17.0 years. The MRI plus conventional mammography group has a median survival of 19.5 years. If this difference is statistically significant, which form of bias may be affecting the results?

Accepted Answer

Lead-time bias

Answer

Recall bias

Answer

Selection bias

Answer

Misclassification bias

Answer

Because this study is a randomized controlled trial, it is free of bias

Question 5

A group of bariatric surgeons are investigating a novel surgically-placed tube that drains a portion of the stomach following each meal. They are interested in studying its efficacy in facilitating weight loss in obese adults with BMIs > 40 kg/m2 who have failed to lose weight through non-surgical options. After randomizing 150 patients to undergoing the surgical tube procedure and 150 patients to non-surgical weight loss options (e.g., diet, exercise), the surgeons found that, on average, participants in the surgical treatment group lost 15% of their total body weight in comparison to 4% in the non-surgical group. Which of the following statistical tests is an appropriate initial test to evaluate if this difference in weight loss between the two groups is statistically significant?

Accepted Answer

Unpaired two-sample t-test

Answer

Kaplan-Meier analysis

Answer

Paired two-sample t-test

Answer

Multiple linear regression

Answer

Pearson correlation coefficient

RCTs — MCQs

RCTs — MCQs

On this page

Practice by Chapter

Want unlimited practice?