Evidence of functional bias in the SF-36

Background:

Measures of self-perceived health are important tools for assessing health, identifying disparities, and evaluating health promotion interventions. The SF-36 is the most widely used measure of self-perceived health in the United States. However, it includes items that appear biased toward an assumption that respondents are unhealthy if they are unable to perform certain functions such as walking or climbing stairs.

Methods:

Healthy adults with and without disabilities were recruited and screened to ensure absence of significant chronic conditions or health problems. Screened participants completed a series of measures, including the SF-36. Differential item functioning analysis was used to identify items that performed differently for a particular disability group in comparison to the nondisabled group, even when overall scores were similar.

Results:

When controlling for each respondent's scores in physical or mental health domains of the SF-36, individuals with spinal cord injuries scored significantly worse than the reference group on eight out of ten items on the Physical Functioning subscale and two items on the Role-Physical subscale. People with sensory disabilities scored significantly worse on one Physical Functioning item and one Role-Emotional item. People with mental health disabilities scored significantly worse on Self-Rated Health and two Role-Emotional items.

Conclusions:

Results indicate that the SF-36 penalizes healthy adults who have functional limitations. For disability and health researchers, it is critically important to disentangle confounding between health and function. Without this distinction, researchers cannot use health measures to accurately assess health disparities or effectiveness of health interventions intended to reduce disparities.