This article develops arguments for the use of decision theory, rather than intuition, to determine the size of trials. It is wrong to expect doctors to ignore personal preferences in favor of clinical experiments unless the trial is capable of showing differences in treatment effect that would influence clinical practice substantially. It follows from our analysis that if delta (the treatment effect that the trial is designed to detect) is sufficient to alter clinical practice, then the alpha and beta errors of a trial should be equal. This applies even if a new treatment is to be compared with conventional therapy or if a treatment with high "costs" is compared with a less invasive or more inexpensive method.