An early application of the principle was presented by Joseph Bell in his report of a controlled trial of whooping cough vaccine initiated in 1936 in Virginia, USA, with professional support from the Norfolk City Union of King’s Daughters Visiting Nurse Association. Bell provides us with an early and remarkable example of the measures he took to establish and maintain fair treatment comparisons (Bell 1941). This entailed including all individuals allocated to one or other of the two comparison groups, whether or not they actually received the treatment assigned to them.