title: Confirmatory factory analysis of the Neck Disability Index in a general problematic neck population indicates a one-factor model.
abstract: BACKGROUND
  The Neck Disability Index frequently is used to measure outcomes of the neck. The statistical rigor of the Neck Disability Index has been assessed with conflicting outcomes. To date, Confirmatory Factor Analysis of the Neck Disability Index has not been reported for a suitably large population study. Because the Neck Disability Index is not a condition-specific measure of neck function, initial Confirmatory Factor Analysis should consider problematic neck patients as a homogenous group.
  
  
  OBJECTIVE
  We sought to analyze the factor structure of the Neck Disability Index through Confirmatory Factor Analysis in a symptomatic, homogeneous, neck population, with respect to pooled populations and gender subgroups.
  
  
  METHODS
  This was a secondary analysis of pooled data.
  
  
  METHODS
  A total of 1,278 symptomatic neck patients (67.5% female, median age 41 years), 803 nonspecific and 475 with whiplash-associated disorder.
  
  
  METHODS
  The Neck Disability Index was used to measure outcomes.
  
  
  METHODS
  We analyzed pooled baseline data from six independent studies of patients with neck problems who completed Neck Disability Index questionnaires at baseline. The Confirmatory Factor Analysis was considered in three scenarios: the full sample and separate sexes. Models were compared empirically for best fit.
  
  
  RESULTS
  Two-factor models have good psychometric properties across both the pooled and sex subgroups. However, according to these analyses, the one-factor solution is preferable from both a statistical perspective and parsimony. The two-factor model was close to significant for the male subgroup (p<.07) where questions separated into constructs of mental function (pain, reading headaches and concentration) and physical function (personal care, lifting, work, driving, sleep, and recreation).
  
  
  CONCLUSIONS
  The Neck Disability Index demonstrated a one-factor structure when analyzed by Confirmatory Factor Analysis in a pooled, homogenous sample of neck problem patients. However, a two-factor model did approach significance for male subjects where questions separated into constructs of mental and physical function. Further investigations in different conditions, subgroup and sex-specific populations are warranted.
authors.affiliation: Faculty of Science, Health and Education, Centre for Healthy Activities, Sport and Exercise, University of the Sunshine Coast, Sippy Downs Dve, Sippy Downs, Sunshine Coast Qld 4556 Australia. Electronic address: cp.gabel@bigpond.com.
authors.forename: Charles Philip
authors.initials: CP
authors.lastname: Gabel
authors.affiliation: Department of Psychiatry and Physiotherapy, Faculty of Medicine, Malaga University, SpainAvenida Estación de el Palo, Málaga, 29017 Spain.
authors.forename: Antonio I
authors.initials: AI
authors.lastname: Cuesta-Vargas
authors.affiliation: Educational and Counseling Psychology, University of Louisville, 2323 S Brook St, Louisville, KY, 40208 USA.
authors.forename: Jason W
authors.initials: JW
authors.lastname: Osborne
authors.affiliation: Faculty of Science, Health and Education, Centre for Healthy Activities, Sport and Exercise, University of the Sunshine Coast, Sippy Downs Dve, Sippy Downs, Sunshine Coast Qld 4556 Australia.
authors.forename: Brendan
authors.initials: B
authors.lastname: Burkett
authors.affiliation: Centre for Medical Research (CMR), Faculty of Medicine, Dentistry and Health Sciences, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Perth, Australia.
authors.forename: Markus
authors.initials: M
authors.lastname: Melloh
mesh_terms: D000328:Adult; D004185:Disability Evaluation; D006233:Persons with Disabilities; D005163:Factor Analysis, Statistical; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D019547:Neck Pain / Q000175:diagnosis* / Q000503:physiopathology; D011594:Psychometrics; D012720:Severity of Illness Index; D011795:Surveys and Questionnaires; D014911:Whiplash Injuries / Q000175:diagnosis* / Q000503:physiopathology
keywords: Clinimetrics; Factor analysis; Measurement; Neck; Outcome