Validation and comparison of the National Eye Institute Visual Functioning Questionnaire-25 (NEI VFQ-25) and the Visual Function Index-14 (VF-14) in patients with cataracts: a multicentre study

Abstract

Purpose: The present study aimed to investigate and compare the psychometric properties of the National Eye Institute Visual Functioning Questionnaire-25 (NEI VFQ-25) and the Visual Function Index-14 (VF-14) in a large sample of patients with cataracts.

Methods: A total of 1052 patients with bilateral age-related cataracts were recruited in the study. Patients with other comorbidities that severely impacted vision were excluded. Participants completed the two questionnaires in random order. Classical test theory and Rasch analyses were used to assess the psychometric properties of the questionnaires.

Results: Complete data were obtained from 899 patients. The mean overall index score on the NEI VFQ-25 was 76.1 ± 19.0, while that on the VF-14 was 46.5 ± 15.0. Cronbach's α-values for the NEI VFQ-25 and VF-14 were 0.89 and 0.95, respectively. Ceiling effects were observed on nine of the 12 subscales in the NEI VFQ-25. The correlation between total scores on the NEI VFQ-25 and VF-14 was moderate (r = 0.600; p < 0.001), and subscales of the NEI VFQ-25 were weakly or moderately correlated with the similar domains on the VF-14. Rasch analysis revealed ordered category thresholds and sufficient person separation for both instruments, while the two questionnaires had critical deficiencies in unidimensionality, targeting and differential item functioning.

Conclusion: Neither the NEI VFQ-25 nor VF-14 is optimal for the assessment of vision-related quality of life in typical Chinese patients with cataracts. The potential deficiencies of the questionnaires should be taken into consideration prior to application of the instruments or interpretation of the results.

Keywords: NEI VFQ-25; Rasch analysis; VF-14; cataract; psychometric properties; vision-related quality of life.

Fig. 1

Category probability curves. The x‐axis of the category probability curve reflects the personal latent characteristic and the logit difference of item difficulty, while the y‐axis reflects the probability of the category being chosen. (A) Category probability curves for the questions with difficulty‐related response options in the National Eye Institute Visual Functioning Questionnaire‐25 (NEI VFQ‐25). The category probability curve was intersecting and ordered. The curve at the extreme left represents ‘stopped doing this because of eyesight’, while the curve at the extreme right represents ‘no difficulty at all’. The response option ‘stopped doing this for other reasons or not interested in doing this’ was treated as missing data and not included in the analysis. (B) Category probability curves for questions with frequency‐related response options in the NEI VFQ‐25, illustrating ordered thresholds. The five curves from left to right represent response categories ranging from ‘all of the time’ to ‘none of the time’. (C) Category probability curves for questions with agreement‐related response options in the NEI VFQ‐25, illustrating ordered thresholds. The five curves from left to right represent response categories ranging from ‘definitely true’ to ‘definitely false’. (D) Category probability curves for the Visual Function Index. Each response option was appropriately used, and category calibration increased in an ordered fashion. The curve at the extreme left represents ‘unable to do’, while the curve at the extreme right represents ‘no difficulty’. The response option ‘not applicable’ was treated as missing data.

Fig. 2

Person–item maps for the National Eye Institute Visual Functioning Questionnaire‐25 (NEI VFQ‐25) (A) and Visual Function Index (B). In each person–item map, participants are displayed to the left of the dashed line, with more able participants located at the top of the map. Items are located to the right of the dashed line, with more difficult items located at the top of the map. The vertical line represents the measure of the variable in linear logit units. ‘M’ markers represent the location of the mean measure. ‘S’ markers are placed one sample standard deviation away from the mean. ‘T’ markers are placed two sample standard deviations away.