“Perfect Health” versus “Disease Free”: The Impact of Anchor Point Choice on the Measurement of Preferences and the Calculation of Disease-Specific Disutilities

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Abstract
Background. During preference testing, some investigators use “perfect health” as the upper anchor point of their measurement scale (“Q scale”), whereas others use “disease free” (“q scale”), which can confound the interpretation and comparison of study results. Methods. We measured current health preferences among 74 patients with cervical spondylotic myelopathy (CSM) on both the Q and q scales using the visual analogue scale (VAS), standard gamble (SG), time tradeoff (TTO), and willingness to pay (WTP). Results. There were significant differences in mean Q and q scale values for the VAS, SG, and WTP (for all,P <0.011); there were no significant differences for mean TTO values (P = 0.592). CSM accounted for 63% to 82% of total disutility, whereas other comorbidities accounted for 28% to 37%. Conclusions. Preferences for CSM differ when measured on the Q and q scales. Caution should be used when comparing and interpreting health values measured on scales with different upper anchors.