Trial analysis by treatment allocated or by treatment received? Origins of 'the intention-to-treat principle' to reduce allocation bias: Part 1

Good and bad effects of treatments are sometimes so dramatic that they can be recognised reliably using informal before-and-after treatment comparisons. Relief of pain after spinal injection of a powerful analgesic or allergic reactions to drugs are examples. Much more usually, important wanted and unwanted effects of treatments are not dramatic and cannot be reliably detected using informal comparisons. In this common situation, detecting real treatment effects entails comparing sufficiently numerous people who have received one of two or more alternative treatments, or by comparing contemporaneously those who have received a treatment with those who have not.