Comparing the measurement properties of visual analogue and verbal rating scales.

PURPOSE: Utilising Rasch analysis on the Keratoconus Outcome Research Questionnaire (KORQ) data, we explored the hypothesis that the KORQ with discrete verbal rating scale (VRS) would demonstrate better psychometric properties and provide less noise in measurement than with a visual analogue scale (VAS). METHODS: The KORQ is a keratoconus-specific patient-reported outcome measure; it has activity limitation and symptoms scales. The KORQ scales with two different rating scales (VAS and a discrete 4-response VRS) were completed by self-administration by people with keratoconus. For each KORQ scale, Rasch analysis-based psychometric properties were compared between the two versions. Rasch analysis was also used to optimise rating scale functioning when disordered thresholds were observed. RESULTS: 118 (mean age ± SD, 46.4 ± 0.4 years) and 169 (45.4 ± 14.7 years) people completed the KORQ with VAS and VRS, respectively. Both scales demonstrated high measurement precision. However, the VAS rating scale was disordered (6 out of 11 categories dysfunctional) and had two misfitting items. Conversely, the VRS had ordered categories and no misfitting items. For the disordered VAS, ordering was achieved only after collapsing 11 categories into four categories. In comparison to the KORQ with VRS, the repaired VAS had lower measurement precision, test information, variance explained by the measure, poor targeting, and reduced measurement range. CONCLUSIONS: The KORQ demonstrated superior psychometric properties when measured using a VRS than with a VAS. This illustrates the advantages of verbal rating scales for a patient-reported outcome measurement over a visual analogue scale.

Development of a Keratoconus-Specific Questionnaire Using Rasch Analysis.

PURPOSE: To develop and validate a keratoconus-specific quality of life (QoL) questionnaire: the Keratoconus Outcomes Research Questionnaire (KORQ). METHODS: The study was carried out in three phases. Phase I: content identifications: items were identified based on an extensive literature review, open-ended patient mail survey, and expert consultations. Each item was scored on a visual analog scale (VAS). Phase II: pilot testing using Rasch analysis. Phase III: testing psychometric properties of the final version of the KORQ. RESULTS: Phase I identified 44 items across 3 different content areas: activity limitation (26), symptoms (20), and convenience (8). The 44-item KORQ was self-administered to 158 people with keratoconus. The 44-item KORQ was multidimensional. Unidimensionality was restored by separating items across three content areas (subscales) as identified in phase I. The activity limitation and symptoms subscales demonstrated adequate measurement precision, but convenience (precision, 1.01) did not. Hence, the convenience subscale was discarded. Rasch analysis revealed that the VAS was disordered. The ordering of the VAS was restored by collapsing categories into 4. An iterative Rasch analysis guided item-removal resulted into a 29-item KORQ (18-item activity limitation and 11-item symptoms). The VAS was replaced by a discrete 4-option labeled categorical rating scale, and it was self-administered by 169 people with keratoconus. Both the subscales demonstrated good psychometric properties. The KORQ scores strongly correlated with visual acuity and contrast sensitivity demonstrating its construct validity. CONCLUSIONS: The 29-item KORQ was a psychometrically robust and valid instrument to assess the impact of keratoconus on activity limitation and symptoms.