Objective-To demonstrate the value of item response theory (IRT) and differential item functioning (DIF) methods in examining a health-related quality of life (HRQOL) 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 cancelled 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 paper 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. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access What is new?• Differential item functioning (DIF) can be detected and is important in health-related quality of life (HRQOL) scale development and for different populations.• Implications of ignoring DIF in scales are reviewed.

Patients preferred, were more satisfied, and less bothered with a 6-month injection regimen for osteoporosis.

This article describes CCFA methods for checking IRT model assumptions and provides suggestions for using these methods in practice. It offers insight into ways information gained through IRT can be applied to evaluate items and aid in scale construction.