In a randomized controlled trial, blinding helps prevent bias from which sources?

• A. Publication bias
• B. Selection bias
• C. Observer and participant bias
• D. Attrition bias

Answer: (C)

Blinding reduces bias by keeping participants and those assessing outcomes unaware of which treatment was given, so expectations don’t color responses or measurements. When participants don’t know their assignment, their symptom reporting and adherence are less influenced by beliefs about effectiveness. When researchers or outcome assessors don’t know the allocation, their observations and judgments are less likely to be biased by expectations. This directly addresses observer bias (bias in measuring or interpreting outcomes) and participant bias (bias in participants’ reporting or behavior). Other biases arise from different processes—publication bias from selective reporting, selection bias from how participants are chosen or allocated, and attrition bias from differential dropout—areas not primarily mitigated by blinding.