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Table 2 Fit parameters of the IVI (and subscales) and SF-36 scales compared to Rasch model requirements.

From: The relative impact of vision impairment and cardiovascular disease on quality of life: the example of pseudoxanthoma elasticum

Parameters Rasch model IVI_O IVI_F IVI_E SF36_C SF-36_PF SF-36_MH
Disordered thresholds No Yes No No No No No
No. of misfitting items 0 4 0 1 4 0 1
Person Separation Index > 0.2 4.07 4.28 2.42 2.66 2.64 2.13
Person Reliability > 0.8 0.94 0.95 0.85 0.88 0.87 0.82
Person-item mean difference < 1 1.15 1.25 1.49 0.30 2.25 1.26
Variance by 1st factor > 50% 64.2% 63.3% 63.1% 40.7% 68.2% 66.5%
PCA (Eigenvalue for 1st contrast) < 2.4 3.9 2.4 1.8 8.3 2.3 2.2
Differential Item Functioning        
   Gender < 1.0 NA None None NA None None
Age group
(≤50, > 50)
< 1.0 NA None None NA None None
   Vision impairment (VI, non-VI) < 1.0 NA None None NA None None
Participant person measure mean ± SD - NA 20.16 ± 8.16 8.82 ± 3.24 NA 19.98 ± 7.58 14.48 ± 6.19
Clinically meaningful cut-off - NA < -4.1, > 4.1 < -1.6, > 1.6 NA < -3.8, > 3.8 < -3.1, > 3.1
  1. IVI_O = Impact of Vision Impairment Original; IVI_F = Impact of Vision Impairment Functional; IVI_E = Impact of Vision Impairment Emotional; PCA = Principle Components Analysis; SD = Standard deviation; SF36_C = SF36 Complete; PF = Physical Functioning scale of the SF-36, MH = Mental Health scale of the SF-36; NA = Not assessed; VI = Vision Impairment
  2. Bolded cells indicate misfiting values compared to the Rasch model requirements