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Table 2 Item description, and estimated of differential item functioning where significant

From: Item response modeling: a psychometric assessment of the children’s fruit, vegetable, water, and physical activity self-efficacy scales among Chinese children

Item

Item Question

CTT

IRM

Mean (SD)

CITC

Difficulty differences

  

Boy - Girla

8-10 yo –

11-13 yob

UW - HWc

UW - OW/OBd

HW - OW/OBe

Fruits self-efficacy (Cronbach’s alpha = 0.86)

12

… eat 1 portion of fruit most times when you eat at a fast food place.

1.51 (0.76)

0.49

     

11

… eat 1 portion of fruit at a fast food place at least one time.

1.55 (0.78)

0.49

−0.47**

−0.30*

 

0.20*

0.21*

10

… eat 1 portion of fruit for dinner at home at least one time.

1.84 (0.89)

0.48

−0.17*

   

0.27*

3

…ask someone in your family to buy 3 fruit at least every week.

2.02 (0.85)

0.64

     

4

… eat 1 portion of fruit at lunch at least one time on a school day.

2.06 (0.87)

0.43

     

6

… eat 1 portion of fruit for lunch most non-school days, including weekends.

2.09 (0.85)

0.59

     

2

…ask someone in your family to buy 3 fruit at least one time.

2.13 (0.83)

0.59

  

0.18*

 

−0.19*

1

…ask someone in your family to buy your favorite fruit every week.

2.14 (0.78)

0.54

0.19*

    

5

… eat 1 portion of fruit for lunch at least one time on a non-school day, including weekends.

2.23 (0.84)

0.59

0.19*

0.18*

   

8

…eat 1 portion of fruit for a snack at home at least 4 days a week.

2.24 (0.83)

0.55

0.23*

    

9

…ask someone in your family to serve 1 fruit instead of your usual dessert for dinner most nights.

2.26 (0.83)

0.52

  

−0.16*

  

7

… eat 1 portion of fruit for a snack at home at least one time.

2.35 (0.79)

0.55

0.36*

0.25*

 

−0.32*

−0.25*

Vegetables self-efficacy (Cronbach’s alpha = 0.85)

6

… eat 3 portions of vegetables at least 4 days a week, even when you are stressed.

1.80 (0.83)

0.61

−0.20*

−0.20*

0.48**

0.45**

 

7

… eat 1 portion of a vegetable most times when you eat at a fast food place.

1.91 (0.84)

0.56

     

2

…cut up 1 portion of a vegetable and eat it with a dip for a snack at least one time.

1.91 (0.85)

0.59

  

0.20*

0.21*

 

4

… ask someone in your family to serve 2 vegetables for dinner most nights.

1.94 (0.84)

0.67

  

−0.17*

−0.28*

 

3

… ask someone in your family to serve 2 vegetables for dinner at least one time.

2.03 (0.85)

0.68

     

8

… eat 1 portion of a vegetable most times when you eat at a cafeteria type restaurant.

2.07 (0.85)

0.55

     

5

…eat 3 portions of vegetables at least 4 days a week.

2.18 (0.84)

0.59

 

0.19*

 

0.27*

0.21*

1

…eat 1 portion of a vegetable at lunch at least one time on a school day.

2.37 (0.80)

0.45

  

−0.46**

−0.44**

 

Water self-efficacy (Cronbach’s alpha = 0. 79)

5

... drink 6 glasses or bottles of water for at least one day.

2.21 (0.85)

0.55

  

0.47**

0.44**

 

4

…drink 6 glasses or bottles of water for at least 4 days a week.

2.26 (0.80)

0.65

  

0.19*

  

3

… drink only water whenever you are thirsty for at least 4 days a week.

2.26 (0.82)

0.55

 

−0.29*

   

1

…drink only water whenever you are thirsty for at least one day.

2.42 (0.77)

0.43

0.25*

 

−0.39*

−0.38*

 

2

… drink 4 glasses or bottles of water for at least one day.

2.48 (0.75)

0.65

 

0.32*

   

Physical Activity self-efficacy (Cronbach’s alpha = 0.91)

11

… be physically active more than 30 min for at least 4 days a week, even when the weather outside is bad.

1.83 (0.82)

0.70

     

6

… ask your friends to be physically active with you more than 30 min for at least 4 days a week.

1.87 (0.85)

0.69

  

−0.21*

  

10

… be physically active more than 30 min for at least 4 days a week when your friends want to do something else.

1.90 (0.84)

0.73

−0.22*

−0.25*

   

8

… be physically active more than 30 min for one day, even when you have lots of other things to do.

1.93 (0.84)

0.74

   

−0.26*

 

7

… be physically active more than 30 min for one day, even when you have homework.

1.94 (0.85)

0.71

     

9

… be physically active more than 30 min for at least 4 days a week, even when you have lots of other things to do.

1.98 (0.85)

0.74

 

−0.25*

   

4

… the ability to play team sports like basketball, soccer or softball really well.

2.05 (0.84)

0.60

−0.71***

  

0.28*

0.38*

5

… ask your friends to be physically active with you more than 30 min for at least one day.

2.20 (0.83)

0.56

 

0.19*

 

0.26*

0.21*

12

… be physically active more than 30 min for most nonschool days, including weekends.

2.26 (0.80)

0.58

     

2

… the ability to do physical activities like running, dancing, bicycling, or jumping rope really well.

2.24 (0.75)

0.58

0.44**

−0.22*

 

−0.42*

−0.50**

3

… the ability to play team sports like basketball, soccer or softball.

2.29 (0.81)

0.54

−0.79***

0.27*

0.31*

0.54**

0.23*

1

… the ability to do physical activities like running, dancing, bicycling, or jumping rope

2.59 (0.65)

0.40

0.79***

0.23*

 

−0.43*

−0.46**

  1. Note: CTT classical test theory, IRM item response model, CITT corrected item total correlations, SD standard deviation, UW underweight, HW healthy weight, OW overweight, OB obesity
  2. IRM difficulty difference was assessed to indicate whether an item performed differently between groups of individuals, including sex, age, and body weight status groups. Specifically
  3. a Positive numbers, easier for girls; Negative numbers, easier for boys
  4. b Positive numbers, easier for 11-13-year-old children; Negative numbers, easier for 8-10-year-old children
  5. c Positive numbers, easier for children with healthy weight; Negative numbers, easier for underweight children
  6. d Positive numbers, easier for overweight/obese children; Negative numbers, easier for underweight children
  7. e Positive numbers, easier for overweight/obese children; Negative numbers, easier for children with Healthy weight
  8. *small effect (difference < 0.426)
  9. **moderate effect (0.426 < difference < 0.638)
  10. ***large effect (difference > 0.638)