Differential item functioning of the Functional Independence Measure in higher performing neurological patients.
OBJECTIVE
When comparing outcomes of the Functional Independence Measure (FIM ) between patient groups, item characteristics of the FIM should be consistent across groups. The purpose of this study was to compare item difficulty of the FIM in 3 patient groups with neurological disorders.


METHODS
Patients with stroke (n=295), multiple sclerosis (n=150), and traumatic brain injury (n=88).


METHODS
FIM scores were administered in each group. The FIM consists of a motor domain (13 items) and a cognitive domain (5 items). Rasch rating scale analysis was performed to investigate differences in item difficulty (differential item functioning) between groups.


RESULTS
Answering categories of the FIM items were reduced to 3 (from the original 7) because of disordered thresholds and low answering frequencies. Two items of the motor domain ("bladder" and "bowel") did not fit the Rasch model. For 7 out of the 11 fitting motor items, item difficulties were different between groups (i.e. showed differential item functioning). All cognitive items fitted the Rasch model, and 4 out of 5 cognitive items showed differential item functioning.


CONCLUSIONS
Differential item functioning is present in several items of both the motor and cognitive domain of the FIM. Adjustments for differential item functioning may be required when FIMdata will be compared between groups or will be used in a pooled data analysis.
Dallmeijer
Annet J
AJ
Department of Rehabilitation Medicine, VU University Medical Center Amsterdam, PO Box 7057, 1007 MB Amsterdam, The Netherlands. a.dallmeijer@vumc.nl
Dekker
Joost
J
Roorda
Leo D
LD
Knol
Dirk L
DL
van Baalen
Bianca
B
de Groot
Vincent
V
Schepers
Vera P M
VP
Lankhorst
Gustaaf J
GJ
D000203:Activities of Daily Living; D000328:Adult; D000368:Aged; D001930:Brain Injuries / Q000503:physiopathology / Q000523:psychology / Q000534:rehabilitation*; D003071:Cognition; D004185:Disability Evaluation; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008875:Middle Aged; D009103:Multiple Sclerosis / Q000503:physiopathology / Q000523:psychology / Q000534:rehabilitation*; D017063:Outcome Assessment, Health Care / Q000379:methods*; D011379:Prognosis; D020127:Recovery of Function; D020521:Stroke / Q000503:physiopathology / Q000523:psychology; D000071939:Stroke Rehabilitation