title: Testing whether patients with diabetes and healthy people perceive the meaning of the items in the Persian version of the SF-36 questionnaire similarly: a differential item functioning analysis.
abstract: It has been rarely studied whether observed disparity in health-related quality-of-life (HRQoL) scores between patients with diabetes and healthy individuals is due to differential item functioning (DIF) or a true difference in the underlying construct. This study aimed to examine DIF in the SF-36 questionnaire and its effect on comparing HRQoL scores between patients with diabetes and healthy people.
  
  
  
  The sample consisted of 230 patients with type 2 diabetes and 642 healthy individuals who filled out the Persian version of the SF-36 questionnaire. To detect DIF across patients with diabetes and healthy individuals, multiple-group multiple-indicator multiple-causes model was used. In addition, item calibration strategy was used to determine whether the effect of item-level DIF was transferred to the scale level.
  
  
  
  Nine out of thirty-six (25 %) items were detected as DIF, of which one item (11 %) was flagged as uniform and eight items (89 %) as non-uniform DIF. Most of the DIF items were detected in the mental health component which includes vitality, perceived mental health and social functioning subscales rather than in physical health component. Moreover, nonsignificant latent mean differences for general health perception and social functioning subscales became significant after DIF calibration.
  
  
  
  The findings of the present study show that patients with diabetes and healthy individuals perceived some items in the SF-36 questionnaire differently. More importantly, in some subscales, the effect of item-level DIF was transferred to the scale level. Consequently, considerable caution should be taken in comparing HRQoL scores between patients with diabetes and healthy individuals.
authors.affiliation: Department of Biostatistics, Shiraz University of Medical Sciences, Shiraz, Iran.
authors.forename: Zahra
authors.initials: Z
authors.lastname: Bagheri
authors.affiliation: Department of Biostatistics, Shiraz University of Medical Sciences, Shiraz, Iran. jafarip@sums.ac.ir.
authors.forename: Peyman
authors.initials: P
authors.lastname: Jafari
authors.affiliation: Department of Biostatistics, Shiraz University of Medical Sciences, Shiraz, Iran.
authors.forename: Marzieh
authors.initials: M
authors.lastname: Mahmoodi
authors.affiliation: Endocrine and Metabolism Research Center, Shiraz University of Medical Sciences, Shiraz, Iran.
authors.forename: Mohammad Hossein
authors.initials: MH
authors.lastname: Dabbaghmanesh
mesh_terms: D000328:Adult; D016022:Case-Control Studies; D003924:Diabetes Mellitus, Type 2 / Q000523:psychology*; D005163:Factor Analysis, Statistical; D005260:Female; D006801:Humans; D007492:Iran; D008297:Male; D008875:Middle Aged; D010465:Perception; D011788:Quality of Life; D011795:Surveys and Questionnaires / Q000145:classification / Q000592:standards*; D014177:Translations
keywords: Differential item functioning; Healthy population; Patients with diabetes; Quality of life; SF-36