This includes two separate issues.

Validating scaling techniques such as the TTO or SG is problematical as it is difficult to observe actual trade-offs between the quality and length of life which correspond with the trade-offs measured by the various scaling instruments. Consequently, validity has been determined primarily by face validity. Some have argued that the standard gamble should be regarded as the gold standard for utility measurement as its use assumes the axioms of von Neumann and Morgenstern. This appears to make the standard gamble results consistent with mainstream economic theory. However, as the axioms have been shown to be empirically incorrect and theoretically defective (Schoemaker 1982; Richardson and Pope 2009), rather, for the reasons outlined by Richardson (1994) and Dolan et al (1996), we have accepted the time trade off as having the greatest prima facie validity.