Differential item functioning impact in a modified version of the Roland-Morris Disability Questionnaire.
OBJECTIVE
To evaluate a modified version of the Roland-Morris Disability Questionnaire for differential item functioning (DIF) related to several covariates.


BACKGROUND
DIF occurs in an item when, after controlling for the underlying trait measured by the test, the probability of endorsing the item varies across groups.


METHODS
Secondary data analysis of two studies of participants with back pain (total n = 875). We used a hybrid item response theory/ logistic regression approach for detecting DIF. We obtained scores that accounted for DIF. We evaluated the impact of DIF on individual and group scores, and compared scores that ignored or accounted for DIF in terms of the strength of association with SF-36 subscale scores.


RESULTS
DIF was found in 18/23 items. Salient scale-level differential functioning was found related to age, education, and employment. Overall 24 participants (3%) had salient scale-level differential functioning. Mean scores across demographic groups differed minimally when accounting for DIF. The strength of association of scores with SF-36 scores was similar for scores that ignored and scores that accounted for DIF.


CONCLUSIONS
The modified version of the Roland-Morris Disability Questionnaire appears to have largely negligible DIF related to the covariates assessed here.
Crane
Paul K
PK
Department of Medicine, University of Washington, Harborview Medical Center, 325 Ninth Avenue, Box 359780, Seattle, WA 98104, USA. pcrane@u.washington.edu
Cetin
Karynsa
K
Cook
Karon F
KF
Johnson
Kurt
K
Deyo
Richard
R
Amtmann
Dagmar
D
D000203:Activities of Daily Living; D000293:Adolescent; D000328:Adult; D001416:Back Pain / Q000175:diagnosis* / Q000503:physiopathology; D004185:Disability Evaluation; D016002:Discriminant Analysis; D005163:Factor Analysis, Statistical; D005260:Female; D006801:Humans; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D017063:Outcome Assessment, Health Care / Q000379:methods*; D011446:Prospective Studies; D011594:Psychometrics / Q000295:instrumentation*; D011788:Quality of Life; D016032:Randomized Controlled Trials as Topic; D018434:Sickness Impact Profile; D012959:Socioeconomic Factors; D011795:Surveys and Questionnaires / Q000592:standards*