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The 39-item Parkinson's Disease Questionnaire (PDQ-39): is it a unidimensional construct?Department of Health Sciences, Lund University and Department of Neurology, Lund University Hospital, Lund, Sweden,Peter.Hagell{at}med.lu.se
Department of Health Sciences, Lund University and Department of Neurosurgery, University Hospital, Lund, Sweden The 39-item Parkinson's Disease Questionnaire, and particularly its summary index (PDQ-39SI) is a widely used patient-reported clinical trial endpoint. A basic assumption when summing items into a total score is that they represent a common variable. We therefore assessed the unidimensionality of the PDQ-39SI using Rasch and confirmatory factor analysis. Both analyses showed model misfit. Adjustment for differential item functioning and disordered response category thresholds did not improve model fit, and residual analyses showed deviation from unidimensionality. These data indicate multidimensionality and challenge the interpretation and validity of PDQ-39SI scores. Clinicians and investigators should use and interpret the PDQ-39SI with caution.
Key Words: dimensionality • outcome measurement • quality of life • Parkinson's disease
This version was published on July
1, 2009 Therapeutic Advances in Neurological Disorders, Vol. 2, No. 4,
205-214 (2009) |
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