Item response theory detected differential item functioning between healthy and ill children in quality-of-life measures

Objective: To demonstrate the value of item response theory (IRT) and differential item functioning (DIF) methods in examining a health-related quality-of-life measure in children and adolescents.

Study design and setting: This illustration uses data from 5,429 children using the four subscales of the PedsQL 4.0 Generic Core Scales. The IRT model-based likelihood ratio test was used to detect and evaluate DIF between healthy children and children with a chronic condition.

Results: DIF was detected for a majority of items but canceled out at the total test score level due to opposing directions of DIF. Post hoc analysis indicated that this pattern of results may be due to multidimensionality. We discuss issues in detecting and handling DIF.

Conclusion: This article describes how to perform DIF analyses in validating a questionnaire to ensure that scores have equivalent meaning across subgroups. It offers insight into ways information gained through the analysis can be used to evaluate an existing scale.