Dimensionality and summary measures of the SF-36 v1.6: comparison of scale- and item-based approach across ECRHS II adults population.
OBJECTIVE
The objective of this study was twofold: 1) to confirm the hypothetical eight scales and two-component summaries of the questionnaire Short Form 36 Health Survey (SF-36), and 2) to evaluate the performance of two alternative measures to the original physical component summary (PCS) and mental component summary (MCS).


METHODS
We performed principal component analysis (PCA) based on 35 items, after optimal scaling via multiple correspondence analysis (MCA), and subsequently on eight scales, after standard summative scoring. Item-based summary measures were planned. Data from the European Community Respiratory Health Survey II follow-up of 8854 subjects from 25 centers were analyzed to cross-validate the original and the novel PCS and MCS.


RESULTS
Overall, the scale- and item-based comparison indicated that the SF-36 scales and summaries meet the supposed dimensionality. However, vitality, social functioning, and general health items did not fit data optimally. The novel measures, derived a posteriori by unit-rule from an oblique (correlated) MCA/PCA solution, are simple item sums or weighted scale sums where the weights are the raw scale ranges. These item-based scores yielded consistent scale-summary results for outliers profiles, with an expected known-group differences validity.


CONCLUSIONS
We were able to confirm the hypothesized dimensionality of eight scales and two summaries of the SF-36. The alternative scoring reaches at least the same required standards of the original scoring. In addition, it can reduce the item-scale inconsistencies without loss of predictive validity.
Grassi
Mario
M
Dipartimento di Scienze Sanitarie Applicate, Sezione di Statistica Medica e Epidemiologia, Università di Pavia, Pavia, Italy. mario.grassi@unipv.it
Nucera
Andrea
A
D000328:Adult; D001315:Australia; D005060:Europe; D005500:Follow-Up Studies; D006306:Health Surveys; D006801:Humans; D008403:Mass Screening / Q000379:methods*; D015999:Multivariate Analysis; D025341:Principal Component Analysis; D011788:Quality of Life; D015203:Reproducibility of Results; D012137:Respiratory System; D012140:Respiratory Tract Diseases / Q000517:prevention & control*; D011795:Surveys and Questionnaires; D014481:United States