Quantitative studies of validity: expanding our understanding of disability.

Purpose: This manuscript seeks to explain why applications of item response theory (IRT) and factor analytic methods provide evidence of validity, and why for the same reasons that they provide evidence of validity, they can advance substantive knowledge.

Methods: A narrative review of the psychometrics literature and disability literature is presented explaining the rationale for the use of quantitative validation methods.

Results: Both the field of psychometrics and the application of psychometric methods in rehabilitation science are expanding rapidly. Logistic IRT models and factor analytic methods are the most commonly used validation tools in rehabilitation.

Conclusions: Many of the available psychometric tools provide evidence of validity, because they are powerful tests of formally specified hypotheses regarding how specific observable traits relate to underlying latent construct. Thus, while the methods for studying psychometric validity have largely been focused on the question of whether or not rehabilitation outcome measures can be used with legitimacy, they also offer a potential research tool for explaining the mechanisms of disability within the framework of the International Classification of Functioning, Disability, and Health.

Implications For Rehabilitation: Quantitative validation of a functional status instrument using latent trait modeling techniques (i.e. IRT and CFA) study the measurement value of an instrument and help to ensure that the instrument provides meaningful information to clinical providers. Latent trait modeling has demonstrated that consistent with the ICF, participation restrictions in those with chronic knee osteoarthritis is driven by activity limitations caused by the knee impairments rather than directly by the knee impairments themselves.