Using Rasch analysis to revisit the validity of the Cataract TyPE Spec instrument for measuring cataract surgery outcomes.

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.