Impact of the scale upper anchor on health state preferences

Some studies of patient preferences use a measurement scale with an upper anchor point of "perfect health" ("Q scale"), whereas others use "disease free" ("q scale"). Different measurement scales can lead to problems with interpreting and comparing study results. In an earlier study of patients with degenerative spine disease, the authors showed systematic differences between preferences measured on the Q v. q scales. They sought to validate the differences in Q and q scale measurements in a separate patient population.

The authors measured preferences for current health in a population of 186 patients with cerebral aneurysms using the standard gamble (SG), time tradeoff (TTO), and willingness to pay (WTP) methods. Values were measured on both the Q and q scales and compared with the Wilcoxon signed-rank test. The authors used an additive utility model to calculate aneurysm-specific disutility.

Q and q scale values were different for the SG (mean values Q: 0.77, q: 0.80, P = 0.034), TTO (Q: 0.79, q: 0.81, P = 0.065), and WTP (Q : $117,600, q: $94,500, P < 0.001). Preference values were consistent with patients valuing perfect health more than aneurysm-free health. Cerebral aneurysms accounted for 43% to 86% of total disutility.

Similar to earlier findings in patients with a degenerative spine condition, this validation study showed that preferences for current health in patients with cerebral aneurysms are different when measured on the Q and q scales. Investigators should be mindful of the impact of the scale's upper anchor point on preference values when conducting and interpreting preference studies.