Differential item functioning related to ethnicity in an oral health‐related quality of life measure

International Journal of Paediatric Dentistry 2010; 20: 435–441 Objective. To assess whether an oral health‐related quality of life (OHRQoL)measure showed differential item functioning (DIF) by ethnicity. Methods. A simple random sample of 12‐ and 13‐year‐old schoolchildren enrolled in the Taranaki District Health Board’s school dental service, New Zealand. Each child (n = 430) completed the Child Perception Questionnaire (CPQ_11‐14) in the dental clinic waiting room, prior to a dental examination. The dataset included age, gender, ethnicity, and deprivation status. The general principle of the analytic plan was that equal scores from each CPQ_11‐14 item were expected from both non‐Mäori and Mäori groups regardless of their ethnic group. Ordinal logistic regression was performed. The dependent variables were the CPQ_11‐14 items. The ethnicity group and each CPQ_11‐14 domain score were the independent variables. Non‐uniform DIF was assessed through adding an interaction term for each CPQ_11‐14 sub‐scale. Results. Non‐uniform DIF was found in two items, one in the Functional Limitations sub‐scale and another in the Social Well‐being sub‐scale. Uniform DIF was found in one item of the Emotional Well‐being sub‐scale. Conclusion. Both non‐uniform and uniform DIF by ethnicity was found in three of 37 items of the CPQ_11‐14 questionnaire, showing it is important to perform DIF analysis when applying OHRQoL measures.