To achieve gold standard validation of an MAU instrument each of the necessary conditions above must be met. However, to interpret the final numbers as utilities which are suitable for calculating QALYs also requires the achievement of the 2 conditions below. Each is an arithmetic consequence of the definition of a QALY as ‘QALYs = (length of life)x(utility of life) where length may be discounted for time preference.

(v) Evidence of a scale ‘interval property’ with respect to preferences. This means for example that any 0.2 interval along the scale (eg 0.3-0.5; or 0.6-0.8) would have the same meaning with respect to the preferred trade-off between life and death.

(vi) The strong interval property. Percentage changes in utilities from an MAU instrument should correspond with the percentage changes in the expected length of life in decisions actually made when a trade-off is possible between the length and quality of life.

For a discussion of the strong interval property see Richardson Working Paper 5 (1990) Cost Utility Analysis: What Should be Measured (also published in Social Science & Medicine (1994)).

For practical reasons evidence with respect to the last two criteria would be hard, though not impossible to obtain. The criterion remains virtually unmentioned in the literature. A review of this reveals that, to date, the majority of validation studies are concerned with convergent validity, ie the correlation of instrument scores with non utility, disease specific instruments or correlation with other utility instruments which have been similarly validated! There has been little attention given to the other requirements above. The AQoL is the only instrument whose descriptive system was constructed using the psychometric principles of instrument construction designed to obtain content validity.