Table 3 Item parameters and values for the SS X ² fit index and LR DIF statistics for the potential new items for the pain interference and fatigue scales

Item Stem

a

b ₁

b ₂

b ₃

b ₄

X ²

d.f.

p

X ²

p

X ²

p

Added to the Pain Interference item bank:

It was hard for me to be away from home because I had pain.

2.14

0.36

0.8

1.63

2.18

118

88

0.020

12.6

0.027

10.2

0.070

It was hard to have fun with friends because I was in pain.

2.79

?0.06

0.48

1.3

2.12

107

81

0.028

8

0.154

11.7

0.040

I needed help walking when I was in pain.

2.18

0.4

1.04

1.87

2.35

84

80

0.358

0.9

0.970

3.4

0.640

I walked carefully when I was in pain.

1.92

?0.41

0.1

1

1.65

171

107

0.001

8.1

0.149

4.1

0.531

I had so much pain I had to stop what I was doing.

2.6

?0.07

0.53

1.61

2.27

82

83

0.524

4.6

0.472

10.7

0.058

My pain was so bad that I needed to take medicine to treat it.

1.9

?0.21

0.28

1.08

1.74

112

101

0.211

5.5

0.356

9.9

0.077

It was hard to do things with my family because I had pain.

2.79

0.08

0.61

1.77

2.33

75

72

0.388

3.4

0.647

8.3

0.138

Added to the Fatigue item bank:

Being tired made it hard for me to remember things.

1.56

?0.2

0.67

2.26

3.05

105

94

0.203

3

0.694

1.9

0.861

I felt tired even when I had not done anything.

1.4

?0.46

0.46

1.87

3

105

102

0.396

4.6

0.469

6.2

0.284