Use of item response theory to link 3 modules of functional status items from the Asset and Health Dynamics Among the Oldest Old study.

OBJECTIVE: To link, by using item response theory (IRT), 3 modules of functional status items in the Asset and Health Dynamics Among the Oldest Old (AHEAD) study.
DESIGN: Secondary data analysis of the functional status items in the AHEAD study. In that study, participants completed a common set of functional status items and were randomly assigned to complete 1 of 2 modules containing different functional status items.
SETTING: A nationally representative panel study of elderly. PARTICIPANTS: US baseline data from 4655 respondents.
INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Nineteen common items and 2 sets of supplemental items that measure functional status.
RESULTS: A 2-parameter model for dichotomous items was used for linking the 3 modules of items. By using this model, both sets of supplemental items were successfully linked to the common items, allowing the placement of all items on the same underlying measure of ability. The small-muscle instrumental activities of daily living were the easiest of all the items for respondents to perform. The item on walking from the Longitudinal Study of Aging was the most difficult for respondents to perform.
CONCLUSIONS: IRT-based linking methods were a useful way to overcome test dependency and to place items on a common metric even if different respondents answer different sets of items. Numerous important design features can degrade linking results and should be attended to in future linking studies. Copyright 2002 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation