Which method is most directly used to reduce selection bias in a randomized trial?

Which method is most directly used to reduce selection bias in a randomized trial?

• A. Randomization
• B. Blinding
• C. Control group
• D. Sample size

Answer: (A)

Random assignment directly prevents researchers from shaping groups based on participant characteristics. When participants are allocated to intervention arms by chance, the likelihood that any particular trait, prognosis, or unmeasured factor predicts group membership becomes about the same across groups. This balancing effect reduces selection bias because group differences at the start are not driven by investigators’ or participants’ preferences or knowledge about prognosis. Often this is strengthened by allocation concealment, which hides upcoming assignments from those enrolling participants, further preventing manipulation of who gets placed into which group. Blinding helps reduce biases that occur after allocation, such as biased assessment or treatment of participants, but it doesn’t prevent how participants are chosen for each group. A control group provides a baseline for comparison, not the mechanism that prevents systematic differences in allocation. Increasing sample size improves the precision of estimates and power, but it doesn’t address the systematic differences that selection bias would introduce.

Random assignment directly prevents researchers from shaping groups based on participant characteristics. When participants are allocated to intervention arms by chance, the likelihood that any particular trait, prognosis, or unmeasured factor predicts group membership becomes about the same across groups. This balancing effect reduces selection bias because group differences at the start are not driven by investigators’ or participants’ preferences or knowledge about prognosis. Often this is strengthened by allocation concealment, which hides upcoming assignments from those enrolling participants, further preventing manipulation of who gets placed into which group.

Blinding helps reduce biases that occur after allocation, such as biased assessment or treatment of participants, but it doesn’t prevent how participants are chosen for each group. A control group provides a baseline for comparison, not the mechanism that prevents systematic differences in allocation. Increasing sample size improves the precision of estimates and power, but it doesn’t address the systematic differences that selection bias would introduce.