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.


METHODS
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.


CONCLUSIONS
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.
Langer
Michelle M
MM
Thurstone Psychometric Laboratory, University of North Carolina, Davie Hall, CB #3270, Chapel Hill, NC 27599-3270, USA. langer@email.unc.edu
Hill
Cheryl D
CD
Thissen
David
D
Burwinkle
Tasha M
TM
Varni
James W
JW
DeWalt
Darren A
DA
D000293:Adolescent; D002648:Child; D002908:Chronic Disease / Q000523:psychology / Q000534:rehabilitation*; D006305:Health Status Indicators; D006801:Humans; D007398:Interpersonal Relations; D015233:Models, Statistical; D011788:Quality of Life; D011795:Surveys and Questionnaires