Physical disability construct convergence across instruments: towards a universal metric.

Objectives: This study examines the stability of a physical disability construct across instruments and samples. The purpose is not to report a formal equating of instrument calibrations, but to indicate whether such an effort would be likely to succeed. Theory. The economics transforming health care from its orientation toward crisis-driven disease reactions to population- and evidence-based preventive health management and individualized disease management demand general scale-free measures of functional independence.

Methods: A new method, pseudo-common item equating, is demonstrated. Similar, but not identical items, from different instruments, calibrated on different samples, are compared.

Data: More than 30 articles presenting Rasch analyses of physical functioning scales were reviewed. Four instruments provided data from ten of these articles, for eleven different calibrations (two instruments are both included in one article).

Results: The final overall average correlation disattenuated for error is .93, with an average of 7 pseudo-common items, and an average p-value of .01, meaning that measures based on these calibrations should be linearly transformable versions of the same metric. Scientific importance. The quantitative stability of different areas of physical functional independence across instruments and samples suggests that the development and deployment of a universal metric is a realizable goal.