SummaryMany clinicians remain unsure of the relevance of measuring quality of life to their clinical practice.In health economics quality of life measures have become the standard means of assessing the results of health care interventions and, more controversially, the means of prioritising funding; but they have many other applications. This article-the first of three on measuring quality of life-reviews the instruments available and their application in screening programmes, audit, health care research, and clinical trials. Using the appropriate instrument is essential if outcome measures are to be valid and clinically meaningful.Interest in measuring quality of life in relation to health care has increased in recent years.' 2 The purpose is to provide more accurate assessments of individuals' or populations' health and of the benefits and harm that may result from health care. The term quality of life misleadingly suggests an abstract and philosophical approach, but most approaches used in medical contexts do not attempt to include more general notions such as life satisfaction or living standards and tend rather to concentrate on aspects of personal experience that might be related to health and health care. Some of the commonly used synonyms for quality of life more accurately convey the content and purpose of measures-health related quality of life, subjective health status, functional status. This is the first of three papers intended to review measurement issues surrounding the use of the growing number of questionnaires and interview based instruments designed to assess health related quality of life.

The design, analysis, and interpretation of studies using measures of quality of life vary according to the context of use. In this paper we are primarily concerned with quality of life measures in clinical trials but our comments are relevant in other contexts. DesignApart from the usual considerations of good study design, particular issues in studies measuring quality of life are the choice of dimensions and the selection of instruments to measure these dimensions. There are also several practical considerations.

The importance of general statistical principles of study design and analysis to quality-oflife assessment in clinical trials is emphasized. Basic methods are reviewed briefly, with reference to three examples. Careful use of standard tools supplemented with context-specific scales is recommended. Problems of weighting and aggregation are discussed; the use of simple weighting schemes supplemented by sensitivity analysis is suggested. Some technical issues are explored, including factorial question structure, components of variance to distinguish mean treatment and patient-specific treatment effects and informative loss to follow-up. Simplicity of design, analysis and presentation are stressed.

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