Item response modeling: an evaluation of the children's fruit and vegetable self-efficacy questionnaire

Perceived self-efficacy (SE) for eating fruit and vegetables (FV) is a key variable mediating FV change in interventions. This study applies item response modeling (IRM) to a fruit, juice and vegetable self-efficacy questionnaire (FVSEQ) previously validated with classical test theory (CTT) procedures. The 24-item (five-point Likert scale) FVSEQ was administered to 1578 fourth graders from 26 Houston schools. The IRM partial credit model indicated the five-point response options were not fully utilized. The questionnaire exhibited acceptable (> 0.70) reliability except at the extremes of the SE scale. Differential item functioning (DIF) analyses revealed no response bias due to gender. However, DIF was detected by ethnic groups in 10 items. IRM of this scale expanded what was known from CTT methods in three ways: (i) areas of the scale were identified that were not as reliable, (ii) limitations were found in the response format and (c) areas of the SE scale levels were not measured. The FVSEQ can be improved by including items at the extreme levels of difficulty. DIF analyses identified areas where IRM can be useful to improve the functioning of measures.