A researcher conducts a randomized controlled trial comparing two antihypertensive drugs. After randomization, it is discovered that the allocation sequence was predictable because sealed envelopes were used but they were held up to light to view the assignment before enrollment. This flaw is best described as:

• A Information bias due to open-label design
• B Confounding due to inadequate randomization
• C Attrition bias due to differential dropout
• D Selection bias due to failure of allocation concealment ✓

Explanation

Allocation concealment prevents foreknowledge of which treatment arm a participant will be assigned to, ensuring that the decision to enroll is not influenced by the expected assignment. Translucent envelopes that can be read under light break allocation concealment, allowing selective enrollment (selection bias). This is distinct from blinding (which reduces performance and detection bias) and occurs at the enrollment stage. Confounding is controlled by randomization itself; attrition bias relates to differential follow-up loss.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.