Psychometric properties of visual functioning index using Rasch analysis

Purpose: The visual functioning index (VFI) was one of the first questionnaires developed using classical test theory to assess outcomes of cataract surgery. However, it was not Rasch-validated. The objective of this study was to examine the psychometric properties of the VFI using Rasch analysis in patients with cataract.

Methods: The 11-item VFI was self-administered to 243 patients (mean age 73.9 years) drawn from a cataract surgery waiting list. We examined the response category thresholds, item fit statistics, differential item functioning and unidimensionality for the VFI and its three subscales.

Results: Category thresholds were ordered. The person separation and reliability were low, indicating the poor discriminatory ability of the VFI. No items misfit but there was suboptimal targeting of item difficulty to patient ability. On the whole the items in the VFI were too easy for the sample. Only one item showed moderate differential item functioning.

Conclusions: The VFI does not meet the stringent requirements of the Rasch model. However adding more items to suit the more able patients with cataract as well as those awaiting second-eye cataract surgery could optimize the VFI.

Key Words: Rasch analysis, cataract, cataract surgery, classical test theory, visual functioning index

Acta Ophthalmologica 2009
Received on 18 November, 2008. Accepted on 29 January, 2009.

Vijaya K Gothwal1,2, Thomas A Wright1, Ecosse L. Lamoureux3,4,5 and Konrad Pesudovs1
1NH&MRC Centre for Clinical Eye Research, Department of Ophthalmology, Flinders Medical Centre and Flinders University of South Australia, Bedford Park, South Australia, Australia
2Meera and L B Deshpande Centre for Sight Enhancement, Vision Rehabilitation Centres, L V Prasad Eye Institute, Hyderabad, India
3Centre for Eye Research Australia, Department of Ophthalmology, University of Melbourne, Victoria, 8002, Australia
4Vision CRC, Sydney, New South Wales, Australia
5Singapore Eye Research Institute, Singapore National Eye Centre, Singapore.

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