Which practice is effective for reducing bias by assignment?

• A. Informed consent.
• B. Blinding.
• C. Randomization.
• D. Quality control.

Answer: (C)

Bias by assignment occurs when the way participants are placed into groups creates differences that distort outcomes. Randomization uses chance to assign participants to the study arms, giving each person an equal chance to be in any group. This tends to balance both known and unknown prognostic factors across groups on average, so observed differences in outcomes are more likely due to the intervention itself rather than preexisting differences. Good randomization is often paired with allocation concealment, which keeps those enrolling participants from predicting upcoming assignments and thus protects against selection bias.

Informed consent is essential for ethical participation but doesn’t change how participants are allocated. Blinding helps prevent bias after allocation by concealing who is in which group from participants or researchers, but it doesn’t address how assignments are made. Quality control focuses on data quality and procedures, not on preventing bias in the assignment process.