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Cataract Symptom Scale: clarifying measurement
V K Gothwal1, T A Wright, E L Lamoureux
1NH&MRC Centre for Clinical Eye Research, Department of Ophthalmology, Flinders Medical Centre, Bedford Park, South Australia, Australia.
Aim:
Previous psychometric evaluation of the Cataract Symptom Scale (CSS) focused on classic assessments of reliability and validity. The 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 between 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 symptoms items did not measure the same construct. The resultant nine-item measure was unidimensional.
Conclusions:
The CSS consists of two separate unidimensional constructs: mobility and visual disability. The reduced nine-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.
