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Table 2 Items with DIF between subgroups of relevant variables

From: Re-evaluating a vision-related quality of life questionnaire with item response theory (IRT) and differential item functioning (DIF) analyses

  Item content Subgroups α β 1 β 2 β 3 β 4 β 5 G2 df p ESΔmax θ
Basic aspects             
7. Eyes getting tired Low vision 2.50 -0.93 -0.65 -0.10 0.32 2.13 15.7 5 0.008 -0.80 -0.80
   Moderate vision loss 2.50 -1.37 -1.23 -0.54 0.10 1.86      
Mobility             
3. Seeing steps or curbs* Low vision 2.38 -0.97 -0.61 -0.01 0.21 1.56 14.2 1 < 0.001 -0.74 0.20
   Moderate vision loss 5.76 -0.96 -0.71 -0.26 -0.02 1.68      
Adjustment             
1. Vision in general Self 1.99 -1.98 -1.44 -0.14 0.38 2.33 19.4 5 0.002 -0.70 -2.00
   Assisted 1.99 -2.47 -2.30 -0.56 -0.09 2.33      
1. Vision in general Male 2.08 -2.05 -1.44 0.04 0.40 1.95 17.9 5 0.003 0.62 2.60
   Female 2.08 -1.83 -1.57 -0.27 0.24 3.37      
12. Unhappy situation in life Male 3.27 -0.74 -0.31 0.56 1.03 3.06 15.4 5 0.009 1.00 5.40
   Female 3.27 -0.39 -0.27 0.51 0.88 7.57      
Reading and fine work            
19. Reading labels AMD 3.77 -1.34 -1.21 -0.94 -0.46 0.68 17.8 5 0.003 0.24 -0.80
   Other 3.77 -1.48 -1.16 -0.63 -0.51 0.68      
24. Using tools Male 1.83 -1.22 -0.92 -0.38 -0.15 1.13 35.2 5 < 0.001 -1.22 -1.00
   Female 1.83 -1.80 -1.58 -1.39 -0.94 1.05      
  1. Person distribution standard normal; α = item discrimination parameter; β = item threshold parameter; G2 = Chi-square statistic; df = degrees of freedom; ESΔmax = maximum difference in expected scores, θ = person parameter (disability level is presented at which the maximum difference in expected scores was found); AMD = age-related macular degeneration.
  2. * Non-uniform DIF; † After deleting item 1, DIF for item 12 was not statistically significant.