Visual Activities Questionnaire: Assessment of subscale validity for cataract surgery outcomes
Received 16 March 2009; received in revised form 11 May 2009; accepted 13 May 2009.
Purpose
To examine the psychometric properties of the Visual Activities Questionnaire (VAQ) and each of its subscales in a modern cataract population using Rasch analysis and if flawed, to revise the VAQ and create a valid measure that maximizes its measurement properties.
Setting
Flinders Eye Centre, Flinders Medical Centre, Adelaide, Australia.
Methods
Patients with cataract in 1 or both eyes drawn from the surgical waiting list were mailed the 33-item VAQ for self-administration. The following were examined for the entire questionnaire and each subscale: whether items measured a single construct (unidimensionality), the behavior of response categories, the ability to differentiate between patients' visual abilities (person separation), matching of item difficulty to participant ability (targeting), and whether items function similarly across subgroups of participants (differential item functioning [DIF]).
Results
The VAQ was completed by 561 patients. Response categories were used as intended. The VAQ discriminated the visual ability of the population (person separation, 4.88) but had suboptimum targeting, misfitting items, significant multidimensionality, DIF, and 4 dysfunctional subscales. Elimination of items causing multidimensionality resulted in a reduced 13-item VAQ that met all validity criteria for satisfactory instrument performance. Only 1 valid subscale (peripheral vision) could be preserved in the 13-item VAQ.
Conclusions
The VAQ in its native form was multidimensional and contained subscales with poor psychometric properties. The revised unidimensional 13-item VAQ was more appropriate for application in cataract outcomes assessment. Ideally, more items should be included to improve the targeting of item difficulty to more able cataract patients.
From NH&MRC Centre for Clinical Eye Research (Gothwal, Wright, Pesudovs), Department of Ophthalmology, Flinders Medical Centre and Flinders University of South Australia, Bedford Park, South Australia; Centre for Eye Research Australia (Lamoureux), University of Melbourne; Vision CRC (Lamoureux), Sydney, Australia; Meera and LB Deshpande Centre for Sight Enhancement (Gothwal), LV Prasad Eye Institute, Hyderabad, India; Singapore Eye Research Institute (Lamoureux), Singapore National Eye Centre, Singapore
Corresponding author: Konrad Pesudovs, PhD, NH&MRC Centre for Clinical Eye Research, Department of Ophthalmology, Flinders Medical Centre, Bedford Park, South Australia, 5042, Australia.
No author has a financial or proprietary interest in any material or method mentioned.
Supported in part by National Health and Medical Research Council, Canberra, Australia, Centre of Clinical Research Excellence grant 264620.
ABSTRACT