Rasch analysis of the Indian vision function questionnaire

Vijaya K Gothwal, Deepak K Bagga, Rebecca Sumalini
British Journal of Ophthalmology 2012, 96 (5): 619-23

AIM: To investigate the psychometric properties of the three scales (general functioning, psychosocial impact, visual symptoms) of the Indian vision function questionnaire (IND-VFQ) using the Rasch measurement model.

METHODS: 236 visually impaired patients referred to vision rehabilitation centres were administered the 33-item IND-VFQ. Rasch analysis was used to investigate the scales for the following properties: precision by person separation (ie, discrimination between strata of patient ability, recommended minimum value 2.0), unidimensionality (ie, measurement of a single construct) and targeting (ie, matching of item difficulty to patient ability).

RESULTS: Only the general functioning scale possessed adequate measurement precision (person separation 3.49). However, it lacked unidimensionality as some items did not contribute towards the measurement of a single construct indicating a secondary dimension. This comprised seven mobility items, which formed a separate valid subscale with good targeting (-0.57 logits). Deleting these items restored unidimensionality but a misfitting item required removal. Following this the 13 items fit and were visual functioning related. However, targeting was suboptimal (-1.13 logits).

CONCLUSIONS: The general functioning scale of the IND-VFQ consists of two separate unidimensional constructs: visual functioning and mobility. Both these Rasch scaled versions with good psychometric properties are effective tools for the assessment of visually impaired patients in India.

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