ABSTRACT
Good face and content validity are both essential characteristics of outcome measurements because they ensure that the measurement identifies the symptoms and illnesses that are important in clinical terms and that are relevant to the aims of the study. Measurements are described as being subjective when they are open to interpretation by the subject or the observer. Examples of subjective measurements include questionnaires that collect information such as symptom severity or frequency, quality of life, or satisfaction with medical services using coded responses or scores. In contrast, objective measurements are collected by instruments that are less easily open to interpretation or to influence by the subject or the observer. In trials to measure the efficacy or effectiveness of an intervention, it is crucial that the main outcome measurement is responsive to essential differences between subjects or to changes that occur within a subject.
