In a randomized controlled trial, allocation concealment is BEST achieved by:

• A Randomizing patients using a computer-generated sequence before enrollment
• B Stratified randomization by age and sex
• C Sealed opaque envelopes prepared by the principal investigator
• D Centralised telephone or web-based randomization system operated by an independent party ✓

Explanation

Allocation concealment prevents investigators from knowing the upcoming assignment before a patient is enrolled, eliminating selection bias. The gold standard is centralised (remote) randomization via an independent telephone or internet system, because the enrolling clinician cannot access the sequence. Sealed opaque envelopes can be tampered with; computer-generated lists alone do not conceal allocation if the investigator can access the list before enrolment. Stratified randomization improves balance but does not itself conceal allocation.

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

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