An Evaluation of the 10-item Vision Core Measure 1 (VCM1) Scale (the Core Module of the Vision-Related Quality of Life scale) Using Rasch Analysis

Purpose: To assess and re-engineer the Vision Core Measure 1 (VCM1) questionnaire in low vision (LV) and cataract participants using Rasch analysis. Methods: 295 participants drawn from a low vision clinic and 181 from a cataract surgery waiting list completed the 10-item VCM1. Unidimensionality, item fit to the model, response category performance, differential item functioning (DIF) and targeting of items to patients were assessed. Category collapsing and item removal were considered to improve the questionnaire. Results: The initial fit of the VCM1 (combined populations) to the Rasch model showed lack of fit (χ 2 = 83.3, df = 50, p = 0.002). There was evidence of DIF between the two populations which could not be resolved. Consequently, each population was assessed separately. Irrespective of the population, disordering of response category thresholds was evident. However, collapsing categories produced ordered thresholds and resulted in fit to the Rasch model for the LV (Total χ 2 = 41.6, df = 30; p =...