A number of scales have been developed that measure the functional and psychosocial impacts of oral disorders. The scores derived from these scales are usually used to compare the ‘oral health- related quality of life’ (OHRQoL) of groups defined by age, gender, socioeconomic status, education and ethnicity. The assumption underlying such comparisons is that of measurement equivalence. That is, it is assumed that the scale and the items comprising the scale function in the same way across groups defined by these sociodemographic variables. If a scale and its items are not equivalent then any differences detected may be an artefact of the measurement process rather than a reflection of actual sub-group differences in the underlying trait or construct being measured by the scale (1). Lack of equivalence may occur when one or more items in a scale is interpreted differently by different groups or there are group variations in the relevance of the concepts the items represent. One way of establishing the equivalence of items across the sub-groups participating in a study is through *Correction added after online publication January 14, 2010: Lindye Foster Page was changed to Lyndie Foster Page. Community Dent Oral Epidemiol 2010; 38: 129–135 All rights reserved Ó 2010 John Wiley & Sons A/S Differential item functioning in a Brazilian–Portuguese version of the Child Perceptions Questionnaire (CPQ 11-14 ) Traebert J, de Lacerda JT, Thomson WM, Foster Page L, Locker D. Differential item functioning in a Brazilian–Portuguese version of the Child Perceptions Questionnaire (CPQ 11-14 ). Community Dent Oral Epidemiol 2010. Ó 2010 John Wiley & Sons A ⁄ S. Abstract – Objective: To determine whether a Portuguese language version of the Child Perceptions Questionnaire for 11–14-year-olds (CPQ 11-14 ) showed differential item functioning (DIF) when compared with the original English language version. Methods: CPQ 11-14 data from a school-based Brazilian study (n = 138) was compared with CPQ 11-14 data collected as part of a school-based study conducted in New Zealand (n = 322). In order to detect DIF, ordinal logistic regression analysis was performed with each CPQ 11-14 item as the dependent variable. The independent variables were language group (English versus Portuguese), the CPQ 11-14 sub-scale score of which the item was a part, and an interaction term for language*sub-scale score. Nonuniform DIF was deemed to be present if the interaction term was significant. Moderate to large uniform DIF was deemed to be present if after removing the interaction term the b coefficient (log odds ratio) for language group was significant and numerically greater than 0.64. Analyses were also undertaken to detect pseudo- DIF. Results: Nonuniform DIF was found in five items and moderate to large uniform DIF in an additional four items. Analyses using ‘purified’ sub-scale scores indicated that little of the DIF detected was pseudo-DIF. A comparison of the language groups using DIF affected and DIF-free overall and subscale CPQ 11-14 scores revealed that the DIF detected had only a marginal effect on the differences between language groups in scores. Conclusion: Oral health-related quality of life questionnaires, particularly those that have been translated, need to be assessed for DIF and its likely impact on group comparisons. Jefferson Traebert 1 , Josimari Telino de Lacerda 2 , W. Murray Thomson 3 , Lyndie Foster Page 3 * and David Locker 4 1 Grupo de Pesquisa em Sau ´ de Bucal Coletiva, Universidade do Sul de Santa Catarina, Tubara ˜o, SC, Brazil, 2 Departamento de Sau ´de Pu ´ blica, Universidade Federal de Santa Catarina, Floriano ´ polis, SC, Brazil, 3 Department of Oral Sciences, University of Otago, Dunedin, Otago, New Zealand, 4 Community Dental Health Services Research Unit, Faculty of Dentistry, University of Toronto, Toronto, ON, Canada Key words: differential item functioning; measurement equivalence; oral health- related quality of life measures; test bias Prof. Traebert, Jefferson Av. Jose ´ Aca ´cio Moreira, 787 Dehon Tubara ˜o Santa Catarina Brazil 88704900, Tel: 55 48 36213363 Fax: 55 48 36213363 e-mail: jefferson.traebert@unisul.br Submitted 18 December 2008; accepted 31 October 2009 doi: 10.1111/j.1600-0528.2009.00525.x 129