Rasch analysis of the short form 8-item Parkinson’s Disease Questionnaire (PDQ-8)
To evaluate the Italian version of the 8-item Parkinson’s Disease Questionnaire (PDQ-8)—a subset of PDQ-39 (a 39-item health-related quality of life instrument for subjects with Parkinson’s Disease [PD])—through classical psychometric techniques and Rasch analysis.
Two convenience samples (100 PD subjects each) were observed consecutively from 2004 to 2006. One group completed the PDQ-8 nested within PDQ-39, the other, the stand-alone PDQ-8.
Once verified that the two independent samples came from the same population and showed consistent item calibrations using differential item functioning analysis, the two groups were combined. Cronbach’s alpha was 0.72. According to Rasch analysis, the response scale of PDQ-8 could be simplified into a 3-category rating scale. After that, all the PDQ-8 items fitted the construct that the scale was intended to measure. Item separation reliability of PDQ-8 was 0.98 and person separation reliability was 0.70. Principal component analysis on the standardized residuals suggested a minor departure in the data from Rasch criteria (multidimensionality) and some marginal inter-item dependency.
The PDQ-8 embedded in the PDQ-39 presented psychometric properties similar to the stand-alone PDQ-8. Our results, while consistent with previous classical psychometric analyses, add information on the meaningfulness of PDQ-8 in people with PD. In particular, a simplification of its rating scale is recommended. Moreover, additional analyses should be performed in order to further check unidimensionality and local dependence, and try to improve item selection and scaling properties of the questionnaire. In order to use the PDQ-8 for clinical decision-making in reference to individuals, its reliability should first be increased.
KeywordsPDQ-8 PDQ-39 Parkinson’s disease Quality of life Psychometrics
- 13.Wright, B. D., & Masters, G. N. (1982). Rating scale analysis. Chicago: Mesa Press.Google Scholar
- 14.Bond, T. G., &, Fox, C. M. (2001). Applying the Rasch model: fundamental measurement in the human sciences. Mahwah, NJ: Lawrence Erlbaum Associates.Google Scholar
- 19.Fahn, S., Elton, R.L.; Members of the UPDRS Development Committee. (1987). Unified Parkinson’s Disease Rating Scale. In S. Fahn, C. D. Marsden, D. Calne, M. Goldstein (eds.), Recent developments in Parkinson’s Disease II (pp. 153–163). Florham Park, NJ: MacMillan Healthcare Information.Google Scholar
- 23.Streiner, D. L., & Norman, G. R. (Eds). (1995). Health measurement scales. A practical guide to their development and use, 2nd edn. Oxford: Oxford University Press.Google Scholar
- 24.Bland, J. M., & Altman, D. G. (1997). Cronbach’s alpha. BMJ, 314 (7080), 572.Google Scholar
- 25.Portney, L. G., & Watkins, M. P. (2000). Foundations of clinical research: Applications to practice, 2nd edn. Upper Saddle River, NJ: Prentice-Hall Health.Google Scholar
- 26.Linacre, J. M. (2004). A user’s guide to Winsteps. Rasch-model computer programs. Chicago, IL. http://www.winsteps.com/aftp/winsteps.pdf. Retrieved 10 March 2007.
- 27.Linacre, J. M., & Wright, B. D. (1987). Item bias: Mantel-Haenszel and the Rasch model. http://www.rasch.org/memo39.pdf. Retrieved 30 December 2007.
- 30.Linacre, J. M. (1994). Sample size and item calibration stability. Rasch Measurement Transactions, 7, 328.Google Scholar
- 31.Nunnally, J. C., & Bernstein, I. H. (1994). Psychometric theory, 3rd edn. New York: McGraw-Hill.Google Scholar
- 33.Lopez, W. (1996). Communication validity and rating scales. Rasch Measurement Transactions, 10, 482–483.Google Scholar