Quality of life (QoL) is an ill-defined term. The World Health Organization (WHO, 1948) has declared health to be "a state of complete physical, mental and social well-being, and not merely the absence of disease". Many other definitions of both "health" and "quality of life" have been attempted, often linking the two and, for QoL, frequently emphasising components of happiness and satisfaction with life. In the absence of any universally accepted definition, some investigators argue that most people, in the western world at least, are familiar with the expression "quality of life" and have an intuitive understanding of what it comprises.
However, it is clear that QoL means different things to different people, and takes on different meanings according to the area of application. To a town planner, for example, it might represent access to green space and other facilities. In the context of clinical trials we are rarely interested in QoL in such a broad sense, but are concerned only with evaluating those aspects that are affected by disease or treatment for disease. This may sometimes be extended to include indirect consequences of disease such as unemployment or financial difficulties. To distinguish between QoL in its more general sense and the requirements of clinical medicine and clinical trials, the term "health-related quality of life" (HRQoL) is frequently used in order to remove ambiguity.
Health-related QoL is still a loose definition. What aspects of QoL should be included? It is generally agreed that the relevant aspects may vary from study to study, but can include general health, physical functioning, physical symptoms and toxicity, emotional functioning, cognitive functioning, role functioning, social well-being and functioning, sexual functioning, and existential issues. In the absence of any agreed formal definition of QoL, most investigators circumvent the issues by describing what they mean by QoL, and then letting the items (questions) in their questionnaire speak for themselves. Thus some questionnaires focus upon the relatively objective signs such as patient-reported toxicity, and in effect define the relevant aspects of QoL as being, for their purposes, limited to treatment toxicity. Other investigators argue that what matters most is the impact of toxicity, and therefore their questionnaires place greater emphasis upon psychological aspects such as anxiety and depression. Yet others try to allow for spiritual issues, ability to cope with illness, and satisfaction with life.
Some QoL instruments focus upon a single concept, such as emotional functioning. Other instruments regard these individual concepts as aspects or dimensions of QoL, and therefore include items relating to several concepts. Although there is disagreement about what components should be evaluated, most investigators agree that a number of the above dimensions should be included in QoL questionnaires, and that QoL is a multidimensional construct. Because there are so many potential dimensions, it is impractical to try to assess all these concepts simultaneously in one instrument. Most instruments intended for health-status assessment include at least some items that focus upon physical, emotional and social functioning. For example, if emotional functioning is accepted as being one aspect of QoL that should be investigated, then several questions could evaluate anxiety, tension, irritability, depression, and so on. Thus instruments may contain many items. Although a single "global" question, such as "How would you rate your overall quality of life?", is a useful adjunct to multi-item instruments, global questions are often regarded as too vague and non-specific to be used on their own. Most of the general questionnaires that we describe include one or more global questions alongside a number of other items covering specific issues. Some instruments place greater emphasis upon the concept of global questions, and the EuroQol questionnaire (Appendix E4) asks a parsimonious five questions before using a single global question that enquires about "your health". Even more extreme is the Perceived Adjustment to Chronic Illness Scale (PACIS) described by Hiirny et al. (1993). This instrument consists of a single, carefully-phrased question that is a global indicator of coping and adjustment: "How much effort does it cost you to cope with your illness?" This takes responses ranging between "No effort at all" and "A great deal of effort".
One unifying and non-controversial theme throughout all the approaches is that the concepts forming these dimensions can be assessed only by subjective measures, and that they should be evaluated by asking the patient. "Proxy" assessments, by a relative or other close observer, are usually employed only if the patient is unable to make a coherent response—for example, some patients who are very young or very old, severely ill, or have mental impairment. Furthermore, many of these individual concepts—such as emotional functioning and fatigue—lack a formal, agreed definition that is universally understood by patients. In many cases the problem is compounded by language differences and some concepts do not readily translate to other tongues. There are also cultural differences regarding the importance of the issues. Single-item questions on these aspects of QoL, as for global questions about overall QoL, are likely to be ambiguous and unreliable. Therefore it is usual to develop questionnaires that consist of multi-item measurement scales for each concept.
This book accepts a broad definition of "quality of life", and therefore discusses the design, application and use of single- and multi-item, subjective, measurement scales. Furthermore, although we are predominantly concerned with health-related QoL, in this book we use the briefer and more common abbreviation, QoL, as a synonym for HRQoL.
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