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Impact of the Scale Upper Anchor on Health State Preferences

Section of Neurosurgery, VA Connecticut Healthcare System, West Haven, Connecticut, Joseph.KingJr{at}va.gov, Department of Neurosurgery, Yale University, New Haven, Connecticut

Joel Tsevat, MD, MPH

Section of Outcomes Research, Division of General Internal Medicine, Department of Internal Medicine, University of Cincinnati Medical Center, Cincinnati, Ohio, Center for Clinical Effectiveness, Institute for the Study of Health, University of Cincinnati Medical Center, Cincinnati, Ohio, Veterans Affairs Medical Center, Cincinnati, Ohio

Mark S. Roberts, MD, MPP

Section of Decision Sciences and Clinical Systems Modeling, Division of General Internal Medicine, Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, Division of General Internal Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania

Background . 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. Methods. 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. Results. 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. Conclusions. 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.

Key Words: aneurysm (cerebral) • preferences • standard gamble • time trade-off • utility • willingness to pay.

This version was published on March 1, 2009

Medical Decision Making, Vol. 29, No. 2, 257-266 (2009)
DOI: 10.1177/0272989X08326148


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