title: Comparing results from long and short form versions of the Parkinson's disease questionnaire in a longitudinal study.
abstract: BACKGROUND
  The purpose of this study was to determine the extent to which summary index scores from the short form Parkinson's Disease Questionnaire (PDQ-8) replicate those from the parent form (PDQ-39) in a longitudinal study.
  
  
  METHODS
  Longitudinal data gained from the PD-MED trial were examined (n = 1867), to determine the extent the PDQ-8 replicates results from the PDQ-39 at baseline and follow up. The sensitivity to change of the PDQ-8 was also compared with that of the PDQ-39. Finally, results on the two measures were compared with those from the Hoehn and Yahr (HY) clinical staging scale.
  
  
  RESULTS
  Results of the Single Index summary score gained from the PDQ-8 were found to closely replicate those gained from the PDQ-39 at each of the three time points. Furthermore at each time point the intraclass correlation coefficient between the two measures was very high (ICC range 0.93-0.96). Similarly, the two measures gave very similar accounts of change (e.g. from baseline to follow up at one year effect sizes were 0.18 for the single index calculated using the PDQ-39, and 0.09 when calculated using the PDQ-8). Similar levels of correlation were found between the two indices when correlated with the HY scale.
  
  
  CONCLUSIONS
  The PDQ-8 closely replicates results gained from the PDQ-39 when calculating single indices. In instances where a single summary score of the impact of PD on self-reported quality of life is needed, it is likely the PDQ-8 will provide reliable and accurate information.
authors.affiliation: Health Services Research Unit, Nuffield Department of Population Health, University of Oxford, Oxford, UK. Electronic address: crispin.jenkinson@dph.ox.ac.uk.
authors.forename: Crispin
authors.initials: C
authors.lastname: Jenkinson
authors.affiliation: Department of Neurology, City Hospital, Sandwell and West Birmingham Hospitals NHS Trust, Birmingham, UK.
authors.forename: Carl
authors.initials: C
authors.lastname: Clarke
authors.affiliation: Clinical Trial Service Unit, Nuffield Department of Population Health, University of Oxford, Oxford, UK.
authors.forename: Richard
authors.initials: R
authors.lastname: Gray
authors.affiliation: Health Services Research Unit, Nuffield Department of Population Health, University of Oxford, Oxford, UK.
authors.forename: Paul
authors.initials: P
authors.lastname: Hewitson
authors.affiliation: Birmingham Clinical Trials Unit, University of Birmingham, Birmingham, UK.
authors.forename: Natalie
authors.initials: N
authors.lastname: Ives
authors.affiliation: Health Services Research Unit, Nuffield Department of Population Health, University of Oxford, Oxford, UK.
authors.forename: David
authors.initials: D
authors.lastname: Morley
authors.affiliation: Birmingham Clinical Trials Unit, University of Birmingham, Birmingham, UK.
authors.forename: Caroline
authors.initials: C
authors.lastname: Rick
authors.affiliation: Cancer Research UK Clinical Trials Unit, School of Cancer Studies, University of Birmingham, UK.
authors.forename: Keith
authors.initials: K
authors.lastname: Wheatley
authors.affiliation: Department of Neurology, University Hospitals Birmingham NHS Foundation Trust, Birmingham, UK.
authors.forename: Adrian
authors.initials: A
authors.lastname: Williams
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D010300:Parkinson Disease / Q000175:diagnosis*; D011594:Psychometrics / Q000295:instrumentation*; D011795:Surveys and Questionnaires / Q000592:standards*
keywords: PDQ single index; PDQ-39; PDQ-8; Parkinson's disease questionnaire; Patient reported outcomes