Double blinding in a trial primarily helps prevent bias in which aspect?

• A. Subject enrollment
• B. Allocation of treatment
• C. Measurement of outcomes
• D. Follow-up completeness

Answer: (C)

Double blinding mainly prevents observer bias in how outcomes are measured or reported. When neither the participant nor the investigator knows which treatment was given, expectations about the treatment cannot color the assessment of results. This is especially important for subjective outcomes, like pain or symptom scales, where measurement relies on judgment. Even for more objective endpoints, the neutrality in assessment helps avoid subtle biases that could creep in if knowledge of assignment influenced interpretation or reporting.

Enrollment and allocation are governed by randomization and allocation concealment, and follow-up completeness is influenced by retention efforts and study design; blinding reduces bias in outcome assessment rather than these other stages.