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Further structural validation of the Long-Term Conditions Questionnaire (LTCQ): formation of the Rasch 8-item LTCQ short-form (LTCQ-8)

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Further structural validation of the Long-Term Conditions Questionnaire (LTCQ): formation of the Rasch 8-item LTCQ short-form (LTCQ-8)

2018 Conference Presentation

Demand measurement and projection United Kingdom

11 September 2018

Further structural validation of the Long-Term Conditions Questionnaire (LTCQ): formation of the Rasch 8-item LTCQ short-form (LTCQ-8)

Laurie Batchelder, University of Kent, United Kingdom

Diane Fox, PSSRU, University of Kent
Caroline Potter, Nuffield Department of Population Health, University of Oxford
Ray Fitzpatrick, Nuffield Department of Population Health, University of Oxford
Julien Forder, PSSRU, University of Kent
Karen Jones, PSSRU, University of Kent
Michele Peters, Nuffield Department of Population Health, University of Oxford

Abstract

Objectives: The aim was to evaluate the structural validity of the 20-item Long-Term Conditions Questionnaire (LTCQ-20) and to form a short scale using Rasch analysis.

Methods: Data were collected from a sample of 1,211 participants with LTCs. Identified participants were invited on the basis of having at least one LTC through either local authorities for a social care cohort (N=294) or primary care practices for a health care cohort (N=917). Participants were mailed survey 1, including the LTCQ, demographic questions, a comorbidities measure, and some validated outcome measures. Respondents were invited to complete a follow-up survey including the LTCQ for assessment of reproducibility. WINSTEPS 3.92.0 software was used for Rasch analysis.
Results: The main assumptions of the Rasch model from the LTCQ-20 were fulfilled, although infit and outfit indices indicated some items showed slight misfit. Misfitted items, items that did not have a preceding set or showed local dependence were removed after iterative analyses, and the LTCQ-8 was formed. The Rasch model for the LTCQ-8 explained 64% variance and a reliability estimate greater than 0.80. Several items in the LTCQ-20 showed differential item function in relation to the number of reported LTCs, age and cohort, but there were fewer in the LTCQ-8. The LTCQ-8 showed excellent test-retest reliability (ICC = 0.92, 95% CI). Spearman’s rho correlations between the LTCQ-20 and the LTCQ-8 were strong across the total sample and various subgroups. Correlations between the LTCQ-8 and all reference measures were moderate to strong, and comparable to correlations found between the LTCQ-20 and these measures.

Conclusion: The LTCQ-20 measures a unidimensional construct, and it is therefore acceptable to use a summed total score. The LTCQ-8 also met the assumption of unidimensionality and had comparable test re-test reliability and construct validity with the LTCQ-20. Additional validation is required in an independent sample.