Classical test theory versus Rasch analysis for quality of life questionnaire reduction
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Classical test theory versus Rasch analysis for quality of life questionnaire reduction Luis Prieto*1, Jordi Alonso2 and Rosa Lamarca2 Address: 1Health Outcomes Research Unit. Eli Lilly and Company, Madrid, Spain and 2Health Services Research Unit. Institut Municipal d'Investigació Mèdica (IMIM). C/ Dr. Aiguader, 80; 08003 Barcelona, Spain Email: Luis Prieto* - [email protected]; Jordi Alonso - [email protected]; Rosa Lamarca - [email protected] * Corresponding author
Published: 28 July 2003 Health and Quality of Life Outcomes 2003, 1:27
Received: 11 April 2003 Accepted: 28 July 2003
This article is available from: http://www.hqlo.com/content/1/1/27 © 2003 Prieto et al; licensee BioMed Central Ltd. This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original URL.
Abstract Background: Although health-related quality of life (HRQOL) instruments may offer satisfactory results, their length often limits the extent to which they are actually applied in clinical practice. Efforts to develop short questionnaires have largely focused on reducing existing instruments. The approaches most frequently employed for this purpose rely on statistical procedures that are considered exponents of Classical Test Theory (CTT). Despite the popularity of CTT, two major conceptual limitations have been pointed out: the lack of an explicit ordered continuum of items that represent a unidimensional construct, and the lack of additivity of rating scale data. In contrast to the CTT approach, the Rasch model provides an alternative scaling methodology that enables the examination of the hierarchical structure, unidimensionality and additivity of HRQOL measures. METHODS: In order to empirically compare CTT and Rasch Analysis (RA) results, this paper presents the parallel reduction of a 38-item questionnaire, the Nottingham Health Profile (NHP), through the analysis of the responses of a sample of 9,419 individuals. Results: CTT resulted in 20 items (4 dimensions) whereas RA in 22 items (2 dimensions). Both instruments showed similar characteristics under CTT requirements: item-total correlation ranged 0.45–0.75 for NHP20 and 0.46–0.68 for NHP22, while reliability ranged 0.82–0.93 and 0.87–94 respectively. Conclusions: Despite the differences in content, NHP20 and NHP22 convergent scores also showed high degrees of association (0.78–0.95). Although the unidimensional view of health of the NHP20 and NHP22 composite scores was also confirmed by RA, NHP20 dimensions failed to meet the goodness-of fit criteria established by the Rasch model, precluding the interval-level of measurement of its scores.
Introduction Several questionnaires have been developed and are currently in extensive use to assess health-related quality of life (HRQOL) [1]. Such instruments may offer satisfactory properties in terms of measurement (i. e. validity and reliability), but their length often limits the exten
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