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Finally, differences among patients regarding the meaning of each item are assessed using differential item functioning (DIF) (Bond & Fox 2001). DIF allows each item calibration to be compared between two groups in order to assess whether group membership affects responses to VFI items. DIF equal to or larger than 0.5 logits was treated as substantial, demonstrating that response probabilities are not explained fully by the latent trait (i.e. visual disability in this case). Descriptive data were analysed using statistical analysis software (SPSS v.15; SPSS, Chicago, Illinois, USA) and p? http://www.selleck.cn/products/Bleomycin-sulfate.html to the Rasch model. The overall fit of the data to the model was poor, indicating that the VFI did not form a valid measure. The real-person separation was 1.02 (model person separation 1.24), with a person separation reliability of 0.51 (Table?3). The mean of patients and items was mistargeted with a mean difference of ?3.20 logits. Cronbach��s alpha was 0.83. Category thresholds were ordered for all the item groups and none of the items misfit. The person-item map for the VFI is shown in Fig.?1. The mean person ability was ?3.20 logits (SD 1.98 logits) (range 1.35 to ?6.04), which is significantly different from a normal distribution (Kolmogorov�CSmirnov Z-test score?=?1.65, p?=?0.008) http://www.selleckchem.com/products/Rapamycin.html and is higher than the average required by the items. However, the mean person ability for patients with bilateral cataract was statistically significantly lower (?2.85 versus ?3.54 logits, independent samples test, F?=?0.02, p?=?0.01) than for those awaiting surgery in the other eye. The item difficulty ranged from ?2.69 to 2.90 logits (Table?4). The http://www.selleckchem.com/products/MG132.html person-item threshold map showed a floor effect for persons and an uneven spread of items across the full range of patients�� scores. Most of the patients had maximum Rasch scaled scores because of no/little difficulty with the tasks. On the whole the items are less difficult for the abilities of the patients, represented by items being located higher than most of the patients in Fig.?1. While the items are placed sufficiently far apart (>?0.50 logits), indicating that there are no overlapping items, only a third of the items are more difficult than the mean item difficulty (mean?=?0). There are few items towards the bottom of the map that would discriminate patients with higher ability. The most difficult item to endorse was related to the quality of distance vision (item no. 2) and the least difficult item related to watching TV (Table?4). Only one item showed DIF: patients ��?70?years rated reading small/large printing types 0.81 logits relatively easier than other items compared to those aged