title: Constantly proving the opposite? A test of CPTO using a broad time horizon and correcting for discounting.
abstract: OBJECTIVE
  An 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.
  
  
  METHODS
  In 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.
  
  
  RESULTS
  We 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.
  
  
  CONCLUSIONS
  Our findings highlight the importance of elicitation methods and context dependencies in QALY measurement and warrant detailed investigation of their influence.
authors.affiliation: iBMG/iMTA, Erasmus University Rotterdam, PO Box 1738, 3000 DR Rotterdam, The Netherlands. attema@bmg.eur.nl
authors.forename: Arthur E
authors.initials: AE
authors.lastname: Attema
authors.forename: Werner B F
authors.initials: WB
authors.lastname: Brouwer
mesh_terms: D000223:Adaptation, Psychological; D000293:Adolescent; D000328:Adult; D002648:Child; D002675:Child, Preschool; D006801:Humans; D007223:Infant; D008875:Middle Aged; D057240:Patient Preference / Q000706:statistics & numerical data; D011788:Quality of Life / Q000523:psychology*; D019057:Quality-Adjusted Life Years; D011795:Surveys and Questionnaires; D055815:Young Adult