Physical disability construct convergence across instruments: towards a universal metric

Area of Science:

• Health outcomes research
• Psychometrics
• Disability studies

Background:

• Healthcare is shifting towards evidence-based and individualized management.
• There is a need for general, scale-free measures of functional independence.
• Existing physical disability instruments may not be directly comparable.

Purpose of the Study:

• To examine the stability of the physical disability construct across different instruments and samples.
• To assess the feasibility of creating a universal metric for functional independence.
• To determine if different physical functioning scales can be equated.

Main Methods:

• A novel method, pseudo-common item equating, was employed.
• Similar, non-identical items from different physical functioning instruments were compared.
• Data from over 30 articles using Rasch analysis of physical functioning scales were reviewed, utilizing eleven calibrations from four instruments.

Main Results:

• An overall average correlation of .93 (disattenuated for error) was found between measures.
• An average of 7 pseudo-common items were used in the comparisons.
• The results indicate that measures from different calibrations are likely linearly transformable versions of the same metric (average p-value = .01).

Conclusions:

• The quantitative stability of physical functional independence across instruments and samples is high.
• The development and implementation of a universal metric for physical disability is a realistic objective.
• This stability supports the move towards more standardized and comparable health outcome measures.