Exploratory factor analysis of PROMIS-29 V1.0, PROMIS Global Health and the RAND SF-36 from chiropractic responders attending care in a practice-based research network

The SF-36 questionnaire is perhaps the most widely used quality of life instrument in the world today, while the PROMIS instruments continue to gain popularity. Given their continued use in chiropractic research and practice, we examined their latent domain structure using exploratory factor analysis (EFA).

To uncover latent structures of a large series of measured variables from the PROMIS-29, PROMIS Global Health and RAND SF-36 domains, we defined a factor analysis model represented by the equation \(X = \mu + \Lambda F +\epsilon\), where \(X = (X_{1} , \ldots ,X_{p} )^{T}\) is the matrix of random vectors corresponding to the domains with a mean of \(\mu\) and the covariance matrix \(\Sigma ,\) \(\Lambda = \{ l_{jk} \}_{pxm}\) denotes the matrix of factor loadings, \(F = (F_{1} , \ldots ,F_{m} )^{T}\) denotes the matrix of unobserved latent variables that influence the collection of domains and \(\epsilon = (_{1} , \ldots ,_{p} )^{T}\) is the vector of latent error terms. The matrix of item responses X was the only observed quantity with restrictions such that variable scores were uncorrelated and of unit variance with the latent errors being independent with the variance vector \(\psi\). The inherited structure of X was expressed simply by \(\Sigma = \Lambda \Lambda^{T} + \psi\). Orthogonal and oblique rotations were performed on the \(\Lambda\) matrix with this equation to improve clarity of the latent structure. Model parameters \(\left( {\mu ,\Lambda ,\psi } \right)\) were optimized using the method of minimum residuals. Each EFA model was constructed with Pearson and Polychoric correlation.

For the PROMIS-29, domains were confirmed to be strongly correlated with Factor 1 (i.e., mental health) or Factor 2 (i.e., physical health). Satisfaction with participation in social roles was highly correlated with a 3rd factor (i.e., social health). For the PROMIS Global Health Scale, a 2-factor EFA confirmed the GPH and GMH domains. For the RAND SF-36, an apparent lack of definable structure was observed except for physical function which had a high correlational relationship with Factor 2. The remaining domains lacked correlation with any factors.

Distinct separation in the latent factors between presumed physical, mental and social health domains were found with the PROMIS instruments but relatively indistinguishable domains in the RAND SF-36. We encourage continued efforts in this area of research to improving patient reported outcomes.