Measuring designates the process by which a system is interacted with in order to represent some of its aspects by means of abstract terms or those that have a merely ideal character, establishing common references that allow comparison with a predetermined pattern and bounded by predetermined margins of uncertainty. Obviously, measurement is an essential component of science; one of its hallmarks, a primary source of knowledge and the essential link between theory and experimentation, allowing them to feed off each other and continuously improve. In the scientific study of any phenomenon, the intervention of the observer and the very fact of the observation disturb the phenomenon studied. For this reason, it is always necessary to take into consideration the degree of disturbance it produces, something that in clinical research can be particularly complex due to the multiplicity and variability of human manifestations. The complexity of the observed subject (human being), given his condition as a thinking, emotional, moral and social entity, multiplies the uncertainty of his responses, both individual and collective, and extreme methodological and ethical demands of the scientist; In addition, the response of each individual to a clinical intervention incorporates multiple other factors unrelated to the effects objectively attributable to the intervention, but which must be duly weighed. For all these reasons, in order to scientifically measure the effect of an intervention in a clinical trial, it is essential to previously define and justify the minimum magnitude of the change that can give rise to an effect that is clinically relevant for the patient and for the observer.