- This is the idea that the rankings or utility scores attached to entities like health states in experiments should be invariant with respect to the procedure used to elicit them. It requires that logically equivalent procedures for expressing preferences should yield identical results in experiments. If a subject stipulates that 20 years of life in full health is equivalent to 40 years of life with an impairment, procedural invariance requires that the subject display the same indifference if asked instead to specify the number of years with the impairment considered to be equivalent to 20 years of good health. Propositions in welfare economics critically depend upon procedural invariance. Unfortunately, empirical research has displayed that people systematically violate procedural invariance.