The 39-item Parkinson's disease questionnaire (PDQ-39) revisited: implications for evidence-based medicine.
Background: The 39 item Parkinson's disease questionnaire (PDQ-39) is the most widely used patient reported rating scale in Parkinson's disease. However, several fundamental measurement assumptions necessary for confident use and interpretation of the eight PDQ-39 scales have not been fully addressed. Methods: Postal survey PDQ-39 data from 202 people with Parkinson's disease (54% men; mean age 70