In the elaborate paper by Sir John Simon, on the History and Practice of Vaccination, presented to Parliament in 1857 and reprinted in the First Report of the Royal Commission, he tells us that the earlier evidence of the value of vaccination was founded on _individual cases_, but that now “from individual cases the appeal is to masses of national experience.” And the marginal reference is, “Evidence on the protectiveness of vaccination must now be statistical.” If this was true in 1857, how much more must it be so now, when we have forty years more of “national experience” to go upon. Dr. Guy, M.D., F.R.S., enforces this view in his paper published by the Royal Statistical Society in 1882. He says: “Is vaccination a preventive of small-pox? To this question there is, there can be, no answer except such as is couched in the language of figures.” But the language of figures, otherwise the science of statistics, is not one which he who runs may read. It is full of pitfalls for the unwary, and requires either special aptitude or special training to avoid these pitfalls and deduce from the mass of figures at our command what they really teach. A commission or committee of enquiry into this momentous question should have consisted wholly, or almost wholly, of statisticians, who would hear medical as well as official and independent evidence, would have all existing official statistics at their command, and would be able to tell us, with some show of authority, exactly what the figures proved, and what they only rendered probable on one side and on the other. But instead of such a body of experts, the Royal Commission, which for more than six years was occupied in hearing evidence and cross-examining witnesses, consisted wholly of medical men, lawyers, politicians, and country gentlemen, none of whom were trained statisticians, while the majority came to the enquiry more or less prejudiced in favour of vaccination. The report of such a body can have but little value, and I hope to satisfy my readers that it (the Majority Report) is not in accordance with the facts; that the reporters have lost themselves in the mazes of unimportant details; and that they have fallen into some of the pitfalls which encumber the path of those who, without adequate knowledge or training, attempt to deal with great masses of figures.