title: Likert and Guttman scaling of visual function rating scale questionnaires.
abstract: OBJECTIVE
  To test the assumptions underlying Likert scoring of visual function questionnaires.
  
  
  METHODS
  Questionnaires were administered to 284 low-vision subjects by telephone. Each subject was administered two of four questionnaires: ADVS, NEI VFQ-25 plus supplement, expanded VAQ, and VF-14.
  
  
  RESULTS
  Z-scores for cumulative frequency of using each rating category across subjects are not linear with rating category rank and items are not the same difficulty for any of the questionnaires. Guttmann coefficients of reproducibility ranged from 57% for the ADVS to 51% for the NEI VFQ-25. Cronbach alphas ranged from 0.92 for the VF-14 to 0.96 for the NEI VFQ; however, inter-item consistency coefficients ranged from 0.24 for the VAQ to 0.45 for the NEI VFQ. Likert scores were significantly correlated between instruments, ranging from 0.66 for NEI VFQ vs ADVS to 0.90 for the VF-14 vs. ADVS.
  
  
  CONCLUSIONS
  The rating scales of all four questionnaires fail to satisfy Likert's assumptions. Also, ratings are probabilistic, rather than deterministic, which means that the Likert model is not valid for these questionnaires. However, Likert scores for all four instruments are intercorrelated, suggesting that they are monotonic with the latent subject trait distributed in the low vision sample.
authors.affiliation: Lions Vision Research and Rehabilitation Center, 6th floor, Wilmer Ophthalmological Institute, Johns Hopkins University School of Medicine, 550 N. Broadway, Baltimore, MD 21205, USA. rmassof@lions.med.jhu.edu
authors.forename: Robert W
authors.initials: RW
authors.lastname: Massof
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D006801:Humans; D008875:Middle Aged; D015233:Models, Statistical; D011338:Probability Theory; D018434:Sickness Impact Profile; D011795:Surveys and Questionnaires; D014787:Vision Tests / Q000379:methods*; D015354:Vision, Low / Q000175:diagnosis*; D019987:Persons with Visual Disabilities / Q000706:statistics & numerical data