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BioMed Research International
Volume 2018, Article ID 1013453, 30 pages
https://doi.org/10.1155/2018/1013453
Research Article

Evaluating the Longitudinal Item and Category Stability of the SF-36 Full and Summary Scales Using Rasch Analysis

1Curtin University, School of Occupational Therapy, Social Work and Speech Pathology, Bentley, Australia
2Monash University, Occupational Therapy Department, Melbourne, Victoria, Australia
3The University of Sydney, Faculty of Health Sciences, Sydney, New South Wales, Australia
4The University of Newcastle, School of Medicine and Public Health, Callaghan, New South Wales, Australia

Correspondence should be addressed to Reinie Cordier; ua.ude.nitruc@reidroc.einier

Received 6 August 2018; Accepted 1 October 2018; Published 4 November 2018

Academic Editor: Adam Reich

Copyright © 2018 Reinie Cordier et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Introduction. The Medical Outcome Study Short Form 36 (SF-36) is widely used for measuring Health-Related Quality of Life (HRQoL) and has undergone rigorous psychometric evaluation using Classic Test Theory (CTT). However, Item Response Theory-based evaluation of the SF-36 has been limited with an overwhelming focus on individual scales and cross-sectional data. Purpose. This study aimed to examine the longitudinal item and category stability of the SF-36 using Rasch analysis. Method. Using data from the 1921-1926 cohort of the Australian Longitudinal Study on Women’s Health, responses of the SF-36 from six waves of data collection were analysed. Rasch analysis using Winsteps version 3.92.0 was performed on all 36 items of the SF-36 and items that constitute the physical health and mental health scales. Results. Rasch analysis revealed issues with the SF-36 not detected using classical methods. Redundancy was seen for items on the total measure and both scales across all waves of data. Person separation indexes indicate that the measure lacks sensitivity to discriminate between high and low performances in this sample. The presence of Differential Item Functioning suggests that responses to items were influenced by locality and marital status. Conclusion. Previous evaluations of the SF-36 have relied on cross-sectional data; however, the findings of the current study demonstrate the longitudinal efficacy of the measure. Application of the Rasch Measurement Model indicated issues with internal consistency, generalisability, and sensitivity when the measure was evaluated as a whole and as both physical and mental health summary scales. Implications for future research are discussed.