Increasingly, clinical trials in neurology are using rating scales of patient-based outcomes to evaluate the effectiveness of therapeutic interventions and, therefore, influence patient care and the expenditure of public funds. To justify this important role in research and clinical practice, these measurement methods must demonstrate that they are rigorous indicators of abstract and unobservable variables such as disability and health-related quality of life.
This chapter, divided into five major sections, is about rating scales for measuring health. The first section examines the history underpinning the science of health measurement and psychometrics, demonstrates that the scientific foundations of this new medical discipline are deeply rooted in education and psychology, and discusses how clinical variables can be rigorously measured. Sections two to four discuss frameworks for evaluating, choosing and developing measures. This approach, rather than recommending a list of instruments, has been chosen because the clinical neurologist is often faced with a choice of instruments. No instrument has universal usefulness, it is therefore important to identify the strengths and weaknesses of individual measures. Section five introduces some of the new developments in health measurement.
Health measurement and psychometrics
Although health measurement as a distinct discipline emerged in the 1980s (Ware et al., 1980; McDowell & Newell, 1987; Streiner & Norman, 1989), it is derived from well-established theories and methods of measurement in the field of social sciences whose origins can be traced to the mid-1800s. The basic scientific principles of measurement were established by mathematical psychologists interested in the human being as a measuring instrument. By studying how people make subjective judgements about measurable physical stimuli (e.g. length, weight, loudness), they developed the science of psychophysics: the precise and quantitative study of how human judgements are made (Guilford, 1954). The investigation of overt responses to physical stimuli requires precise methods, referred to as psychophysical methods, for presenting the stimuli and for measuring responses (Nunnally, 1959). The work of psychophysicists seems far removed from health measurement. It established the fundamental principles of subjective measurement which are as equally relevant to judgements about health as to judgements about physical stimuli. The psychophysicists demonstrated that: subjective judgement is a valid approach to measurement; humans make judgements about abstract comparisons in an internally consistent manner; and accurate judgements can be made on ratio rather than simple ordinal scales.