Should the SCOPA-COG be modified? A Rasch analysis perspective.

BACKGROUND AND PURPOSE

The SCales for Outcomes in PArkinson's disease-Cognition (SCOPA-COG) is a specific measure of cognitive function for Parkinson's disease (PD) patients. Previous studies, under the frame of the classic test theory, indicate satisfactory psychometric properties. The Rasch model, an item response theory approach, provides new information about the scale, as well as results in a linear scale. This study aims at analysing the SCOPA-COG according to the Rasch model and, on the basis of results, suggesting modification to the SCOPA-COG.

METHOD

Fit to the Rasch model was analysed using a sample of 384 PD patients.

RESULTS

A good fit was obtained after rescoring for disordered thresholds. The person separation index, a reliability measure, was 0.83. Differential item functioning was observed by age for three items and by gender for one item.

CONCLUSIONS

The SCOPA-COG is a unidimensional measure of global cognitive function in PD patients, with good scale targeting and no empirical evidence for use of the subscale scores. Its adequate reliability and internal construct validity were supported. The SCOPA-COG, with the proposed scoring scheme, generates true linear interval scores.