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Likert and Guttman scaling of visual function rating scale questionnaires.

Robert W Massof1

  • 1Lions 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

Ophthalmic Epidemiology
|December 14, 2004
PubMed
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Likert scoring assumptions are not met by four visual function questionnaires in low-vision patients. Despite this, Likert scores show monotonic relationships, suggesting they capture a similar underlying trait.

Area of Science:

  • Ophthalmology
  • Psychometrics
  • Health Outcomes Research

Background:

  • Visual function questionnaires are crucial for assessing patient-reported outcomes in low-vision individuals.
  • Likert scoring is a common method for analyzing data from such questionnaires, assuming specific properties of the rating scales.

Purpose of the Study:

  • To rigorously evaluate whether the rating scales of four commonly used visual function questionnaires adhere to the underlying assumptions of Likert scoring.
  • To determine the validity of applying Likert models to analyze visual function data.

Main Methods:

  • Four visual function questionnaires (ADVS, NEI VFQ-25, expanded VAQ, VF-14) were administered to 284 low-vision subjects via telephone.
  • Statistical analyses included assessing linearity of rating category use, item difficulty, Guttmann coefficients of reproducibility, Cronbach's alpha, and inter-item consistency.

Related Experiment Videos

Main Results:

  • The rating scales across all four questionnaires failed to meet the linearity and equal difficulty assumptions of Likert scoring.
  • Guttmann coefficients of reproducibility were suboptimal, ranging from 51% to 57%.
  • While Cronbach's alphas were high (0.92-0.96), inter-item consistency coefficients were low (0.24-0.45).
  • Likert scores were significantly and highly correlated between instruments (0.66-0.90).

Conclusions:

  • The Likert model is not strictly valid for these visual function questionnaires due to violated assumptions and probabilistic ratings.
  • Despite theoretical limitations, the intercorrelation of Likert scores suggests they are monotonically related to the underlying latent trait of visual function in the low-vision population.
  • These findings highlight the need for careful interpretation of Likert-based scores from visual function questionnaires.