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Table 3 Item parameter estimates and fit statistics

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

  Item content per dimension α β 1 β 2 β 3 β 4 β 5 X2 df p
  Basic aspects          
6. Seeing moving objects 2.06 -1.21 -0.66 0.13 0.66 2.55 19.6 20 0.49
7. Eyes getting tired 2.61 -1.25 -0.96 -0.29 0.25 2.10 17.0 17 0.45
8. Seeing the television 2.18 -1.69 -1.18 -0.34 0.21 1.98 17.1 18 0.52
9. Glare (dazzled by lights) 1.81 -2.17 -1.80 -1.20 -0.51 1.76 24.8 19 0.17
10. Getting right amount light 2.12 -1.83 -1.26 -0.37 0.22 2.32 13.0 20 0.88
  Mobility          
2. Night vision inside house 1.66 -1.42 -0.88 0.32 0.85 3.16 24.6 22 0.31
3. Seeing steps or curbs 3.56 -1.04 -0.68 -0.09 0.17 1.55 13.9 20 0.84
4. Depth/distance perception 2.41 -0.80 -0.45 0.09 0.51 1.98 23.3 25 0.56
15. Getting around outdoors 5.26 -1.07 -0.75 -0.14 0.22 1.72 7.3 18 0.99
16. Crossing a road with traffic 3.35 -1.11 -0.69 -0.25 0.12 1.52 17.7 22 0.73
  Adjustment          
11. Understand eye condition 1.26 -0.94 -0.47 0.22 0.68 3.48 17.0 21 0.71
12. Unhappy situation in life 3.23 -0.63 -0.37 0.51 0.93 3.22 10.5 21 0.97
13. Frustration with doing tasks 3.41 -1.06 -0.69 0.02 0.38 2.44 19.3 19 0.44
14. Visiting friends and family 1.37 -0.44 -0.13 0.52 0.89 2.99 16.5 21 0.74
  Reading and fine work          
17. Reading large print 2.14 -0.14 0.19 0.85 1.17 2.10 23.2 27 0.68
18. Reading newspaper/books 3.98 -1.42 -1.06 -0.61 -0.26 0.92 23.7 25 0.54
19. Reading labels 3.50 -1.79 -1.44 -0.94 -0.54 0.79 13.2 22 0.93
20. Reading letters and mail 5.02 -1.28 -0.93 -0.36 0.02 0.98 21.9 22 0.47
21. Finding out the time 1.91 -0.50 -0.16 0.59 1.13 2.27 25.8 29 0.64
22. Writing 3.55 -1.04 -0.71 -0.17 0.17 1.06 21.2 27 0.78
23. Reading own handwriting 3.17 -0.81 -0.41 0.11 0.51 1.31 24.5 26 0.55
  1. Person distribution standard normal; α = item discrimination parameter; β = item threshold parameter;
  2. X2 = Chi-squared item fit statistic; df = degrees of freedom.