Psychometric properties of visual functioning index using Rasch analysis.
OBJECTIVE
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.
Gothwal
Vijaya K
VK
National Health and Medical Research Council Centre for Clinical Eye Research, Department of Ophthalmology, Flinders University of South Australia, Adelaide, Australia.
Wright
Thomas
T
Lamoureux
Ecosse L
EL
Pesudovs
Konrad
K
D000368:Aged; D000369:Aged, 80 and over; D002387:Cataract Extraction; D005260:Female; D006801:Humans; D008297:Male; D051346:Mobility Limitation; D011184:Postoperative Period; D011594:Psychometrics / Q000379:methods*; D011932:Reading; D012380:Role; D012919:Social Behavior; D011795:Surveys and Questionnaires / Q000592:standards*; D016896:Treatment Outcome; D014786:Vision Disorders / Q000150:complications / Q000503:physiopathology / Q000523:psychology; D014785:Vision, Ocular