How discriminating are discriminative instruments?
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How discriminating are discriminative instruments? Matthew Hankins1,2,3 Address: 1King's College London, Department of Psychology (at Guy's), Institute of Psychiatry, London, UK, 2Department of Primary Care & Public Health, Brighton & Sussex Medical School, Brighton, UK and 3Brighton & Sussex University Hospitals NHS Trust, Royal Sussex County Hospital, Brighton, UK Email: Matthew Hankins - [email protected]
Published: 27 May 2008 Health and Quality of Life Outcomes 2008, 6:36
doi:10.1186/1477-7525-6-36
Received: 31 October 2007 Accepted: 27 May 2008
This article is available from: http://www.hqlo.com/content/6/1/36 © 2008 Hankins; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract The McMaster framework introduced by Kirshner & Guyatt is the dominant paradigm for the development of measures of health status and health-related quality of life (HRQL). The framework defines the functions of such instruments as evaluative, predictive or discriminative. Evaluative instruments are required to be sensitive to change (responsiveness), but there is no corresponding index of the degree to which discriminative instruments are sensitive to cross-sectional differences. This paper argues that indices of validity and reliability are not sufficient to demonstrate that a discriminative instrument performs its function of discriminating between individuals, and that the McMaster framework would be augmented by the addition of a separate index of discrimination. The coefficient proposed by Ferguson (Delta) is easily adapted to HRQL instruments and is a direct, non-parametric index of the degree to which an instrument distinguishes between individuals. While Delta should prove useful in the development and evaluation of discriminative instruments, further research is required to elucidate the relationship between the measurement properties of discrimination, reliability and responsiveness.
Background The McMaster framework [1] defines the functions of health status instruments as evaluative, predictive, or discriminative. Evaluative instruments measure longitudinal change, typically the effects of treatment. Predictive instruments are used classify individuals against an external criterion and are intended for diagnostic, prognostic or screening purposes. Discriminative instruments are used to quantify differences between individuals when no external criterion exists, typically in cross-sectional studies. These functional definitions have been widely adopted as the methodological basis for the measurement of healthrelated quality of life (HRQL) [2]. The validity of a HRQL instrument depends primarily on the instrument measuring the correct aspect of HRQL [3].
This is usually demonstrated by appropriate correlations with other measures [4]. Beyond
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