Using Rasch Analysis to revisit the validity of the cataract TyPE Spec: an instrument for measuring cataract surgery outcomes


Purpose: To assess the psychometric properties of the Cataract TyPE Specification (Spec) questionnaire using the Rasch model.

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: STwo 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.

J Cataract Refract Surg. 2009;35:1509-17.
Accepted for publication March 30, 2009.

Vijaya K. Gothwal, PhD, Thomas A. Wright, Ecosse L. Lamoureux, PhD, Konrad Pesudovs, PhD
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.


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