At the conclusion of a double-blinded, randomized clinical trial of propranolol hydrochloride, but before unblinding, the patients and clinic personnel were asked to guess the treatment group assignment of each patient. While 79.9% of the patients receiving propranolol correctly identified their treatment group assignment, 57.2% of the patients receiving placebo incorrectly guessed that they were also in the propranolol group. No specific mechanism was identified to explain why more patients receiving propranolol were better able to guess their group assignment. Clinic physicians correctly identified the group assignment of 69.9% of the patients receiving propranolol and 68.8% of the patients receiving placebo. Clinic coordinators correctly identified the group assignment of 67.1% of the patients receiving propranolol and 70.6% of the patients receiving placebo. For clinic personnel, heart rate level and heart rate change seem to be the mechanisms employed to identify their patients' treatment assignment.
Byington RP, Curb JD, Mattson ME. Assessment of Double-blindness at the Conclusion of the β-Blocker Heart Attack Trial. JAMA. 1985;253(12):1733–1736. doi:10.1001/jama.1985.03350360059018