The author has been at the centre of a number of classical trials of drugs and antibiotics as well as of vaccines. Streptomycin completely changed the whole pattern of treatment of tuberculosis, antihistamines did not do much for the common cold; both the positive and the negative findings were important scientific conclusions based on this blend of statistical method and clinical appreciation. Probably no therapeutic agent has been so fruitful with hope or so fraught with dangers as cortisone. The Medical Research Council under the author’s guidance has enabled a precise evaluation of potentiality to be achieved. In relation to vaccines, too, the contribution of the statistical method has been vital—illustrated here by trials of whooping cough vaccine, BCG, and influenza vaccine. The book ends with a tribute to an earlier practitioner of logic—John Snow— the man who banished cholera. It is a cautionary tale. ‘. . . the whole of Snow’s case rested upon circumstantial evidence, almost entirely upon statistical observations and relationships. Even the not so stubborn were allergic to that kind of evidence—and are still allergic to it’.