title: Differential item and test functioning methodology indicated that item response bias was not a substantial cause of country differences in mental well-being.
abstract: OBJECTIVE
  Establishing the cross-cultural equivalence of the mental well-being construct, as measured with the Warwick-Edinburg Mental Well-being Scale (WEMWBS), by studying potential construct validity biases in two countries with previously reported score differences.
  
  
  METHODS
  We compared the WEMWBS total scores and item responses in Scotland (N = 779) and Catalonia (N = 1,900) general population samples. To assess whether the questionnaire spuriously favored higher scores in Catalonia, we tested for differential item functioning (DIF) by applying ordinal logistic regression on Item Response Theory scores. DIF was tested with likelihood ratio tests and standard effect measures (McFadden Pseudo R(2), >0.13; relative parameter change, >5%), and differential test functioning (DTF) was tested by plotting differences between full-test and purified (i.e., without DIF items) score estimates.
  
  
  RESULTS
  Catalonia showed higher levels of mental well-being than Scotland (Cohen d = 0.84). Three of 14 WEMWBS items showed small amounts of DIF. DIF did not accrue to DTF, as shown by intraclass correlation coefficient (ICC, 0.999) and case-by-case differences (maximum, 0.12 SD) between total and purified scores. Population differences remained mainly constant across sociodemographics and health outcomes.
  
  
  CONCLUSIONS
  The WEMWBS measures a distinct well-being construct that is stable across countries, implying that Scotland and Catalonia populations are effectively different in the distribution of mental well-being. This result adds to previous psychometric information and supports WEMWBS as a valid unbiased measures for individual and cross-cultural comparisons.
authors.affiliation: CIBER Epidemiología y Salud Pública (CIBERESP), Instituto de Salud Carlos III, Mechor Fernandez de Almagro, 3-5, 28029, Madrid, Spain; Health Services Research Group, IMIM-Institut Hospital del Mar d'Investigacions Mèdiques, Carrer del Doctor Aiguader, 88, PRBB building, 08003 Barcelona, Spain.
authors.forename: Carlos G
authors.initials: CG
authors.lastname: Forero
authors.affiliation: CIBER Epidemiología y Salud Pública (CIBERESP), Instituto de Salud Carlos III, Mechor Fernandez de Almagro, 3-5, 28029, Madrid, Spain; Health Services Research Group, IMIM-Institut Hospital del Mar d'Investigacions Mèdiques, Carrer del Doctor Aiguader, 88, PRBB building, 08003 Barcelona, Spain.
authors.forename: Núria D
authors.initials: ND
authors.lastname: Adroher
authors.affiliation: Department of Statistics and Epidemiology, Warwick Medical School, University of Warwick, Coventry CV4 7AL, UK.
authors.forename: Sarah
authors.initials: S
authors.lastname: Stewart-Brown
authors.affiliation: CIBER Epidemiología y Salud Pública (CIBERESP), Instituto de Salud Carlos III, Mechor Fernandez de Almagro, 3-5, 28029, Madrid, Spain; Health Services Research Group, IMIM-Institut Hospital del Mar d'Investigacions Mèdiques, Carrer del Doctor Aiguader, 88, PRBB building, 08003 Barcelona, Spain.
authors.forename: Pere
authors.initials: P
authors.lastname: Castellví
authors.affiliation: Agència de Salut Pública de Catalunya, Subdirecció General de Drogodependències, Carrer Roc Boronat 81-95, 08005 Barcelona, Spain.
authors.forename: Miquel
authors.initials: M
authors.lastname: Codony
authors.affiliation: CIBER Epidemiología y Salud Pública (CIBERESP), Instituto de Salud Carlos III, Mechor Fernandez de Almagro, 3-5, 28029, Madrid, Spain; Health Services Research Group, IMIM-Institut Hospital del Mar d'Investigacions Mèdiques, Carrer del Doctor Aiguader, 88, PRBB building, 08003 Barcelona, Spain.
authors.forename: Gemma
authors.initials: G
authors.lastname: Vilagut
authors.affiliation: Agència de Salut Pública de Catalunya, Subdirecció General de Drogodependències, Carrer Roc Boronat 81-95, 08005 Barcelona, Spain.
authors.forename: Anna
authors.initials: A
authors.lastname: Mompart
authors.affiliation: Agència de Salut Pública de Catalunya, Subdirecció General de Drogodependències, Carrer Roc Boronat 81-95, 08005 Barcelona, Spain.
authors.forename: Ricard
authors.initials: R
authors.lastname: Tresseres
authors.affiliation: Agència de Salut Pública de Catalunya, Subdirecció General de Drogodependències, Carrer Roc Boronat 81-95, 08005 Barcelona, Spain.
authors.forename: Joan
authors.initials: J
authors.lastname: Colom
authors.affiliation: Institute of Neuropsychiatry and Addictions (INAD), Parc de Salut Mar, Passeig Maritim 25-29, 08003 Barcelona, Spain.
authors.forename: José I
authors.initials: JI
authors.lastname: Castro
authors.affiliation: CIBER Epidemiología y Salud Pública (CIBERESP), Instituto de Salud Carlos III, Mechor Fernandez de Almagro, 3-5, 28029, Madrid, Spain; Health Services Research Group, IMIM-Institut Hospital del Mar d'Investigacions Mèdiques, Carrer del Doctor Aiguader, 88, PRBB building, 08003 Barcelona, Spain; Universitat Pompeu Fabra (UPF), Plaça de la Mercè, 10-12. 08002 Barcelona, Spain. Electronic address: jalonso@imim.es.
authors.forename: Jordi
authors.initials: J
authors.lastname: Alonso
mesh_terms: D000328:Adult; D000368:Aged; D015982:Bias; D003431:Cross-Cultural Comparison; D005260:Female; D054624:Health Status Disparities; D006801:Humans; D008297:Male; D008603:Mental Health; D008875:Middle Aged; D012606:Scotland; D013030:Spain; D011795:Surveys and Questionnaires / Q000592:standards*; D055815:Young Adult
keywords: Bias; Construct validity; Cross-cultural studies; Happiness; Health outcomes; Inequalities; Measurement invariance; Patient-reported outcomes measurement; Positive mental health; Positive psychology