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 implied by the various scaling instruments. 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, the axioms have been shown to be empirically incorrect and theoretically defective (Schoemaker 1982; Richardson and Pope 2009). Because of this history there has been little discussion of the question ‘how should we evaluate utilities’ or, more generally, ‘how should we decide upon the measurement units used for QoL’.

As used in CUA, ‘utility’ needs two interval properties:

1. the weak (conventional) ‘interval property’ is that an interval or scale (eg an increase of 0.2 from 0.1 to 0.3 or from 0.8 to 1.0) has the same meaning with respect to preferences.
2. the ‘strong interval property’ is that the preference for a 10 percent increase in the utility index would be the same as the preference for a 10 percent increase in the number of life years. This follows from the definition that QALY = utility_*(life years). The left hand side of this equation is equally affected by a 10 percent increase in either of the right hand side variables. For a discussion see Richardson (1994), Cost Utility Analysis: What should be measured’, Social Science and Medicine.