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Should the SCOPA‐COG be modified?: a Rasch analysis perspective
Author(s) -
Forjaz M. J.,
FradesPayo B.,
RodriguezBlazquez C.,
Ayala A.,
MartinezMartin P.
Publication year - 2010
Publication title -
european journal of neurology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.881
H-Index - 124
eISSN - 1468-1331
pISSN - 1351-5101
DOI - 10.1111/j.1468-1331.2009.02791.x
Subject(s) - rasch model , cog , reliability (semiconductor) , psychometrics , construct validity , cognition , item response theory , scale (ratio) , content validity , measure (data warehouse) , clinical psychology , medicine , psychology , developmental psychology , computer science , psychiatry , data mining , artificial intelligence , thermodynamics , physics , power (physics) , quantum mechanics
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.