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Aim To investigate the psychometric properties of the three scales (general functioning, psychosocial impact, visual symptoms) of the Indian vision function questionnaire (INDeVFQ) using the Rasch measurement model. Methods 236 visually impaired patients referred to vision rehabilitation centres were administered the 33item INDeVFQ. 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 INDeVFQ 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.

Glaucoma is the leading cause of irreversible blindness worldwide after cataract and accounts for 10% of the world's blind.1 Although patients with acute angle closure glaucoma are symptomatic, those with other forms of glaucoma (at least in the early stages) usually do not experience many symptoms, thereby, resulting in the disease often going unnoticed with possible progression. In some cases, patients with fluctuating levels of intraocular pressure may experience blurred vision and see haloes around lights. The Glaucoma Symptom Scale (GSS) was developed to assess ophthalmic symptoms experienced by patients with glaucoma.2 The GSS comprises 10 ocular symptoms, of which six are non-visual and four are visual. The non-visual symptoms include ‘burning/smarting/stinging’, ‘tearing’, ‘dryness’, ‘itching’, ‘soreness/tiredness’, and ‘feeling of something in the eye’. The visual symptoms include ‘blurry/dim vision’, ‘hard to see in daylight’, ‘hard to see in darkness’, and ‘halos around lights’. The GSS is unique in that it assesses symptoms as compared with visual functioning by other glaucoma-specific questionnaires. Like most questionnaires in ophthalmology, the GSS was also developed using traditional psychometric methods, that is, the classical test theory (CTT).3 The limitations of CTT have been well acknowledged. A major shortcoming of CTT pertains to its scoring assumptions: Likert or summary scoring in which the scores are calculated from …