Donabedian (1985) defined health outcome as a change in patients' current and future health status that can be attributed to antecedent health care. Outcome indicators include information on avoidable mortality, standardized mortality ratios, hospital readmission or retreatment rates (analysed in relation to cause), other service use indicators, laboratory investigations and other diagnostic tests, morbidity, case severity, adverse reactions, complications, the technical success of the treatment where quantifiable, symptom relief, pain, cost-effectiveness, patient satisfaction, and, increasingly, broader indicators of health status or health-related quality of life. Clinical indicators of outcome are no longer sufficient, particularly in view of the debate about whether to survive in a vegetative state is no better than death, or even worse (Jennett, 1976). It is important that the research design enables a comparison to be made between the outcome of the intervention group and a control group, in order to ensure that the outcome obtained would not have occurred anyway (i.e. without the intervention). Baselines are not necessarily static and require monitoring via controls.

The term quality of life is often used but seldom defined. It is a broader concept than health status, and takes social well-being into account. Health-related quality of life can be defined as optimum levels of mental, physical, role (e.g. work, school, parent, carer, etc.) and social functioning, including relationships. It also encompasses perceptions of health, fitness, life satisfaction and well-being. It should also include some assessment of the patient's level of satisfaction with treatment, outcome and health status and with future prospects. It is an abstract concept, comprising diverse areas.