SYMPOSIUM ON QUALITY OF LIFE IN CANCER PATIENTSAssessing the Clinical Significance of Single Items Relative to Summated Scores
Section snippets
DEFINITION OF TERMS
Confusion surrounding the issue of single-item vs summated scores is exacerbated by the terms involved (eg, item, construct, tool, scale, instrument, questionnaire, index). The literature has spawned a plethora of terms that are used interchangeably and can sometimes be misleading. When we began writing this article, we found the terms surrounding single and multiple items a surprising barrier, but one that needed to be surmounted before moving forward. Some terms, for example, tool and
GENERAL CONCEPTS: SINGLE-ITEM SCORE AND SUMMATED SCORE
In some research settings, QOL is operational as a simple and singular concept. For example, one of the European Organization for the Research and Treatment of Cancer Quality of Life Questionnaire 30 core items (EORTC QLQ-C30)2 asks, “How would you rate your overall quality of life during the past week?” The response categories range on a 7-point ordinal scale from “very poof' to “excellent.” The Medical Outcomes Study 36-Item Short-Form Health Survey (SF-36)3 includes the question, “In
THEORETICAL FRAMEWORK: THE RESEARCH QUESTION DRIVES THE CHOICE OF INDEX
Selection of measures of QOL, including the issues surrounding single vs multiple domains and questions, must be motivated by a conceptual fit with the theory driving the assessment measure and the reliability and validity of the index in the population of interest. A priori identification of specific research objectives and their relationship to a theoretical framework can help identify and settle challenges, points of dispute, and logistic problems.
For example, if the purpose of the
ASSESSING THE QUALITY OF A QOL INDEX
A reliable and valid index is necessary to detect meaningful differences between groups and meaningful changes between groups or within groups over time. Evidence of the psychometric robustness of a measure accumulates over time and is never truly complete. Validation data may include the ability to discriminate between groups at a particular point and the ability to detect change over time within a group or between groups. The former is generally established in the first phase of index
MODIFYING INDICES
In the understandable desire to minimize respondent burden, an intuitively appealing approach is to take individual items from existing QOL indices. The putative advantage of such an approach is that one starts with a pool of items whose interrelationships and psychometric performance are known. However, such economies must be undertaken with great care and scientific rigor to ensure the veracity of the subsequent findings. Some researchers eschew such an approach entirely. Two complementary
ANALYTICAL IMPLICATIONS AND INTERPRETATION
The choice of a single index or multiple indices to evaluate QOL, as well as the choice among indices derived from single or multiple items, has a great impact on data analysis and the interpretation of results. A single-global-item index value or composite score has the apparent advantage of both simplified analysis and interpretation. There is 1 answer and no need to address the issues of multiple comparisons. However, these advantages may be outweighed by the inability of a single item to
PRACTICAL CONSIDERATIONS AND GUIDELINES FOR IMPLEMENTATION
The decision whether to use a QOL profile composed of multi-item indices, an abbreviated version of such a profile, or an index composed of single-item measures hinges on a number of interrelated issues of a conceptual, substantive, and practical nature. Frequently, tension mounts between the desire for a reasonably high level of measurement precision driven by the research question and the need for efficiency in carrying out the assessment.
The degree of respondent, staff, and institutional
CONCLUSIONS
The singular conclusion that can be drawn from this examination of single-item vs multi-item index scores is that there is no definitive answer for all situations. There is a place for each approach in the varied and complex world of QOL research. Either approach could provide an easy linkage to the determination of clinical significance for a QOL assessment.
The primary motivating issue is the research question at hand. A careful a priori examination of the research question, theoretical
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This project was supported in part by Public Health Service grants CA25224, CA37404, CA15083, CA35269, CA35113, CA35272, CA52352, CA35103, CA37417, CA63849, CA35448, CA35101, CA35195, CA35415, and CA35103.
Individual reprints of this article are not available. The entire Symposium on the Clinical Significance of Quality-of-Life Measures in Cancer Patients will be available for purchase as a bound booklet from the Proceedings Editorial Office at a later date