Purpose: To determine whether the VF-11 is a valid scale to measure visual functioning in an Asian population with vision impairment.
Methods: Participants from the Singapore Malay Eye Study (SiMES) took part. Visual functioning was assessed by using the VF-11 (a modified version of VF-14 for an Asian population). Rasch analysis was performed on 618 participants with presenting visual acuity < 6/12 in the better eye.
Results: Disordered thresholds were initially evident, indicating that the categories were difficult to discriminate and required category collapsing (from 5 to 4) for nine items. The removal of two misfit items related to driving resulted in a fit of the VF-9 data to the Rasch model (chi(2) = 50.5, df = 27, P = 0.005). There were no more misfit items. The person separation reliability value was 0.82 which demonstrates that the VF-9 has sufficient ability to discriminate between at least two groups of participants with different levels of visual functioning. The VF-9 significantly differentiated patients stratified by visual acuity demonstrating adequate criterion validity. All items were free of differential item functioning, and there was no evidence of multidimensionality. Targeting of person ability and item difficulty was suboptimal, although this is inevitable in a population-based survey where most people would not be disabled.
Conclusions: Although the Rasch-modified VF-9 scale achieved fit to the Rasch model, its suboptimal targeting suggests that the instrument does not have the range of items to assess the impact of vision impairment across the severity spectrum of vision loss in this population.