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Rasch Analysis of the Independent Mobility Questionnaire
Eva K Fenwick1, Fleur O'Hare, Lil Deverell
1*PhD †MPhil(Med) ‡MEd §BOT Center for Eye Research Australia, University of Melbourne, Royal Victorian Eye and Ear Hospital, Melbourne, Victoria, Australia (EKF, FO'H, LD, LNA, CDL, SMc, RHG, RPF); and Australian College of Optometry, University of Melbourne, National Vision Research Institute Melbourne, Victoria, Australia (SAB).
Purpose:
The Independent Mobility Questionnaire (IMQ) assesses participants' perceived ability for independent mobility. However, it has not been validated in a severely visually impaired population. The aim of this study was to explore the IMQ's psychometric properties in participants with severe visual impairment.
Methods:
This was a cross-sectional study of 40 participants with retinitis pigmentosa (better eye visual acuity <20/200 and/or visual field <10%). The key psychometric properties of the IMQ were examined using Rasch analysis, including precision, targeting, and item fit. Construct validity was assessed by testing the correlation between the IMQ and the Mobility and Independence subscale of the Impact of Vision Impairment questionnaire (Pearson correlation coefficient, r). Criterion validity was also assessed.
Results:
The IMQ had excellent precision (Person Separation Index, 3.01) with the capacity to distinguish at least four strata of participant ability, and item difficulty was well targeted to participant ability (difference between mean person and item measures, -0.21). Items 34, 35, 21, and 14 displayed misfit (infit MnSq >1.4); however, given our sample size restrictions, these items were not removed from the analysis. The IMQ had good construct validity (moderate correlation with the Impact of Vision Impairment Mobility subscale, r = 0.595, p < 0.05) but did not demonstrate criterion validity.
Conclusions:
The psychometric properties of the IMQ were promising. Our findings are useful for researchers evaluating the effectiveness of novel treatment technologies on mobility in a severely visually impaired population from the participant's perspective. However, further validation studies in larger samples are required to confirm our results.
