Aim: Previous psychometric evaluation of the Cataract Symptom Scale (CSS) focused on classic assessments of reliability and validity. Our aim was to investigate the psychometric properties of the CSS using the Rasch measurement model.
Methods: 243 patients drawn from the Flinders Eye Centre cataract surgery waiting list self-administered the CSS. Rasch analysis was used to investigate the following properties of the CSS: measurement a single construct (unidimensionality), discrimination between strata of patient ability (person separation) and targeting of item difficulty to person ability.
Results: The CSS discriminated four strata of patients. However, some items did not contribute towards measurement of a single construct indicating a secondary dimension. This comprised three mobility items, which formed a separate valid subscale. Elimination of these items resulted in the CSS being a unidimensional measure. However, further item deletion was required as some more items did not appear to measure the same construct. The resultant 9-item measure was unidimensional.
Conclusions: The CSS consists of two separate unidimensional constructs: mobility and visual disability. The reduced 9-item measure has good psychometric properties and is unidimensional. The CSS is essentially a measure of visual disability and not cataract symptoms as it is claimed to be.
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