Constantly Proving The Opposite? A test of CPTO using a broad time horizon and correcting for discounting

Abstract

PurposeAn important assumption underlying the quality-adjusted life year (QALY) model is that people trade off life years against health in the same proportion irrespective of the number of remaining life years. This is known as the constant proportional trade-offs (CPTO) condition. Previous studies have produced mixed empirical evidence about the validity of CPTO. This paper is the first to test CPTO using the time trade-off (TTO) method for a broad time horizon.MethodsIn a sample of 83 students, we use a choice based TTO protocol to elicit TTO scores for back pain, using ten different gauge durations ranging between 1 and 46 years. The TTO scores are corrected for discounting, which is elicited by means of the direct method.ResultsWe find average TTO scores varying between 0.72 and 0.81. Although the scores do not differ much for different durations in absolute terms, some differences are significant, rejecting CPTO, with and without correcting for discounting. No clear relationship between TTO scores and gauge duration is found. An anchoring and rounding heuristic to some extent explains our results.ConclusionsOur findings highlight the importance of elicitation methods and context dependencies in QALY measurement and warrant detailed investigation of their influence.

Deriving Time Discounting Correction Factors for TTO Tariffs

“…As such, it generates (very) low discounting estimates as well, contrary to applications of the DM in student samples, where much higher discount rates were reported (Attema and Brouwer, 2012a;Attema et al, forthcoming b). Hence, we recommend the development of a toolkit to elicit discounting of future health outcomes that is easier to grasp for the general population.…”

Time trade-off: one methodology, different methods

“…The total time frame, composed of disease duration and lead or lag time, if applicable, is important because TTO values are often found to vary with it [11, 17, 18], notwithstanding the fact that the QALY model predicts them to be independent of the time frame, as implied by the condition of constant proportional trade-offs (CPTO). However, no systematic effect appears from empirical studies.…”

“…However, no systematic effect appears from empirical studies. A tendency for TTO values to decrease with time frame is found, but a lot of mixed evidence and even studies reporting an increasing relationship also exist, making it hard to reach definitive conclusions [17]. Most TTO studies use a 10- or 20-year time frame [2], less often actuarial life expectancy is used [18, 19], and sometimes respondents’ own life expectation is used [20–22].…”

Deriving Time Discounting Correction Factors for Tto Tariffs

“…We deliberately used states that were also included in the health state valuation task, to prevent a distortion in the correction of health state utilities for discounting that would arise if subjects would differentially discount different health states. Attema and Brouwer () reported empirical evidence for such a pattern. The TTO scores for state 32211 [32313] were corrected with the discounting results using both state 21111 [22222] and state 33333.…”