Using Rasch analysis to revisit the validity of the Cataract TyPE Spec instrument for measuring cataract surgery outcomes
Received 27 January 2009; accepted 30 March 2009.
Purpose
To assess the psychometric properties of the Cataract TyPE Specification (Spec) questionnaire using the Rasch model.
Setting
Flinders Medical Centre, Adelaide, Australia.
Methods
The 12-item Cataract TyPE Spec questionnaire was self-administered to patients drawn from the cataract surgery waiting list. The questionnaire and its 5 subscales were assessed for fit to the Rasch model. Response category performance, item-fit targeting, unidimensionality (using principal components analysis), and differential item functioning were assessed. A shortened version (11 items) was tested for criterion validity by determining correlation with a global rating of vision question.
Results
Two hundred ninety-four patients responded to the questionnaire. The response categories for each question functioned as intended. Person-separation reliability was high (0.90). Deletion of 1 misfitting item (nighttime driving) improved overall model fit. The principal components analysis of the residuals demonstrated unidimensionality for the 11-item Cataract TyPE Spec and 2 subscales. However, items were targeted to a less able population. Only 2 subscales (near vision and glare) were valid. There was a good statistically significant correlation between the Likert-scored global rating of vision and the Rasch-scaled Cataract TyPE Spec score (r = −0.66, P<.0001), suggesting good criterion validity.
Conclusions
With minor modifications, the Cataract TyPE Spec questionnaire and its glare and near-vision subscales were good measures of visual functioning in the cataract patient. Additional items to suit the more able, including patients having second-eye surgery, could improve the measurement properties.
From the 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, the Centre for Eye Research Australia (Lamoureux), Department of Ophthalmology, University of Melbourne, Victoria, and Vision CRC (Lamoureux), Sydney, Australia; Meera and LB Deshpande Centre for Sight Enhancement (Gothwal), Vision Rehabilitation Centres, L.V. Prasad Eye Institute, Hyderabad, India; Singapore Eye Research Institute (Lamoureux), Singapore National Eye Centre, Singapore
Corresponding author: Konrad Pesudovs, 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, Centre of Clinical Research Excellence Grant 264620, Canberra, Australia.
ABSTRACT