Differential item functioning in a Brazilian-Portuguese version of the Child Perceptions Questionnaire (CPQ)

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.

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 beta coefficient (log odds ratio) for language group was significant and numerically greater than 0.64. Analyses were also undertaken to detect pseudo-DIF.

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.

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.