The `non observables are banned' position states that any behaviour that cannot be
directly observed and confirmed by an independent observer is unworthy of analysis. In the
development of the SIP, Bergner et al. (1976a) reported deciding that of a feeling state,
clinical, and performance conception of an individuals own health state appraisal, only the
last of these, the performance conception, was suitable. The feeling state conception was
ruled out on the grounds of being inaccessible to external validation, and the clinical
conception was seen as unsuitable as it required medical interpretation and hence was
reliant on the definitions of physicians and not the person concerned. The performance
conception was adopted as it could be based on respondent report, but could also be
easily observed and reported by an untrained observer, and also allowed easy comparison
between different diseases and dysfunctions.