Examination of the Measurement Equivalence of the Functional Assessment in Acute Care MCAT (FAMCAT) Mobility Item Bank Using Differential Item Functioning Analyses

Objective: To assess differential item functioning (DIF) in an item pool measuring the mobility of hospitalized patients across educational, age, and sex groups.

Design: Measurement evaluation cohort study. Content experts generated DIF hypotheses to guide the interpretation. The graded response item response theory (IRT) model was used. Primary DIF tests were Wald statistics; sensitivity analyses were conducted using the IRT ordinal logistic regression procedure. Magnitude and impact were evaluated by examining group differences in expected item and scale score functions.

Setting: Hospital-based rehabilitation.

Participants: Hospitalized patients (N=2216).

Interventions: Not applicable.

Main outcome measures: A total of 111 self-reported mobility items.

Results: Two linking items among those used to set the metric across forms evidenced DIF for sex and age: "difficulty climbing stairs step-over-step without a handrail (alternating feet)" and "difficulty climbing 3-5 steps without a handrail." Conditional on the mobility state, the items were more difficult for women and older people (aged ≥65y). An additional 18 items were identified with DIF. Items with both high DIF magnitude and hypotheses related to age were difficulty "crossing road at a 4-lane traffic light with curbs," "jumping/landing on one leg," "strenuous activities," and "descending 3-5 steps with no handrail." Although DIF of higher magnitude was observed for several items, the scale-level effect was relatively small and the exposure rate for the most problematic items was low (0.35, 0.27, and 0.20).

Conclusions: This was the first study to evaluate measurement equivalence of the hospital-based rehabilitation mobility item bank. Although 20 items evidenced high magnitude DIF, 5 of which were related to stairs, the scale-level effect was minimal; however, it is recommended that such items be avoided in the development of short-form measures. No items with salient DIF were removed from calibrations, supporting the use of the item bank across groups differing in education, age, and sex. The bank may thus be useful to assist clinical assessment and decision-making regarding risk for specific mobility restrictions at discharge as well as identifying mobility-related functions targeted for postdischarge interventions. Additionally, with the goal of avoiding long and burdensome assessments for patients and clinical staff, these results could be informative for those using the item bank to construct short forms.