Validity and Reliability of the German Vocal Fatigue Index in Adults With Voice Disorders

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Abstract
Purpose: Vocal fatigue–related symptoms, frequent in patients with voice disorders, are reliably quantifiable with the Vocal Fatigue Index (VFI) in several languages. The main aim of this work was to investigate the content and construct validity of the German Vocal Fatigue Index (VFI-D) by comparison with the German Voice Handicap Index 9 international (VHI9i) and the Vocal Tract Discomfort Scale (VTD). Moreover, the scale and retest reliability were investigated. Method: In a prospective cohort study at two tertiary care hospitals, 93 adults with all types of voice complaints filled out the VFI-D, the VHI-9i, and the VTD. Eighty-one adults ( M age = 53.5 years, SD = 15.7) completed them one time, and additional 12 patients ( M age = 53.8 years, SD = 15.6) did a test–retest, completed them twice at an interval of 1 week. The validity was assessed by Spearman's rho and Mokken scaling and the reliability by a factor analysis and a two-tailed t test. Results: Mokken scaling has proven the VFI-D as one-dimensional questionnaire (one cluster, r = .95). All VFI-D items could be assigned to one of three factors according to principal component and principal axis factor analysis. Although Factor 1 related best to the VHI9i (∅ r s = .76) and Factor 2 to the VTD (∅ r s = .74), Factor 3 correlated moderately with the VHI9i ( r s = .34) and weakly with the VTD ( r s = .25). Results from the first and second VFI-D completion agreed strongly ( t = −1.78, p = .10). Conclusions: In this study, the content and construct validity of the VFI-D were confirmed. Also, both scale reliability with identical factors as those in the original study and retest reliability were verified. The VHI9i and VTD were highly indicative of VFI-D Factors 1 and 2, respectively. No correlation between Factor 3 and the other questionnaires was determined. Future research should evaluate which characteristics are specific to vocal fatigue versus general voice disorder–related symptoms.

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