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Table 4 Item parameters and values for the SS X 2 fit index and LR DIF statistics for the potential new items for the physical functioning: upper extremity and mobility scales

From: Using item response theory to enrich and expand the PROMIS® pediatric self report banks

   

DIF between

 

Item parameters

S-X2fit index

Boys and Girls

Ages

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 Upper Extremity item bank:

          

I could pick up coins from a table.

2.94

 

-3.23

-2.66

-1.87

10

8

0.247

3.8

0.291††

2.9

0.406††

I could wash my hair by myself.

2.14

 

-2.84

-2.37

-1.61

18

22

0.731

1.3

0.861

5.4

0.253

I could put beads on a string.*

1.95

?3.32

?3.04

?2.41

?1.56

30

23

0.151

6.3

0.283

5.9

0.212

I could comb my hair by myself.

2.43

?3.25

?2.8

?2.59

?1.65

19

14

0.160

3.9

0.269††

5

0.286

I could thread a needle.*

1.54

-2.14

-2.02

-1.43

-0.57

50

41

0.162

9.7

0.083

12.1

0.034

Added to the Mobility item bank:

          

I could jump up and down.

2.93

-2.3

-2.06

-1.7

-1.16

73

43

0.003

4.1

0.539

2.5

0.774

Not added to the Mobility item bank (DIF):

            

I could get in and out of a chair on my own.

3.46

-3.06

-2.79

-2.38

-1.78

32

16

0.010

3.3

0.657

24.6

0.001

Not added to the Mobility item bank (LD and DIF):

          

I could run three miles without stopping.

1.51

-0.52

0.04

1.04

1.74

52

57

0.678

14.8

0.011

7.6

0.176

  1. Notes: The scale for the item parameters is set such that the distribution of physical functioning in the reference population (represented by the NC portion of the sample) is standardized, mean 0 variance 1, as is conventional for reporting IRT parameters. d.f. =5 for all DIF X2 statistics except those marked with , for which d.f. = 4, and those marked with ††, for which d.f. = 3. Values in bold indicate test statistics and p values that are significant at p < .05 after adjustment for multiple comparisons.
  2. *Marked as “enemy items” with each other, and with “I could put toothpaste on my toothbrush by myself” from the original item pool.