A guiding principle in human experimental research which states that, in order for it to be ethical to assign individuals to an exposure (or the lack of an exposure, in the case of the placebo arm of a trial), there must be genuine confidence that a treatment may be worthwhile to justify administering it to some individuals, balanced by genuine reservations about the treatment to justify withholding the treatment from others. (Aschengrau & Seage, p. 155)