Purpose: To compare the discriminatory ability and sensitivity to change of the standard summative score of the Visual Function Index (VF)-14 with two alternative Rasch-based scoring systems.
Methods: A total of 4335 prospective patients with cataracts completed the VF-14 before surgery and 3 months after surgery. Rasch analysis was applied to the VF-14 patient responses before surgery and the VF-14 joint patient responses before and after surgery. To study the discriminatory ability, the VF-14 patient responses were grouped according to the preoperative visual acuity (VA) and the presence of ocular morbidities besides cataracts. For analysis of the sensitivity to change, the overall mean change in VF-14 scores was calculated after surgery, and the patients were grouped according to the presence of other ocular morbidities, postoperative VA gain, and satisfaction with the surgical outcome. The relative precision (RP) index and the effect size were used to compare the different scoring systems.
Results: Rasch analysis confirmed the unidimensional structure of the VF-14. All items and scales adjusted well to the model (fit indices range, 0.71-1.34). The RP index for discrimination by ocular morbidity was 0.82 and by preoperative VA level, 1.02. Regarding sensitivity to change, the RP was 2.68 based on ocular morbidity and 1.78 with samples grouped by postoperative VA gain.
Conclusions: For longitudinal studies in which change is the relevant outcome, Rasch scores should be used, rather than the traditional score. However, for cross-sectional studies, both scoring systems were similarly precise.