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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.