2018 Conference Presentation
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.