500 likes | 575 Views
Extending Medical Preference Models to Include Lifetime Goals. Gordon Hazen Northwestern University INFORMS Pittsburgh, November 2006. Outline of talk. QALYs/ Problems with QALYs Health quality versus life quality: Extrinsic goals Revising the QALY assumptions Survival-duration surrogates
E N D
Extending Medical Preference Models to Include Lifetime Goals Gordon Hazen Northwestern University INFORMS Pittsburgh, November 2006
Outline of talk • QALYs/ Problems with QALYs • Health quality versus life quality: Extrinsic goals • Revising the QALY assumptions • Survival-duration surrogates • Filling gaps in the QALY model • Utility over health profiles • Example decision analysis • Proportionate-duration surrogates • Open issues
Outline of talk • QALYs/ Problems with QALYs • Health quality versus life quality: Extrinsic goals • Revising the QALY assumptions • Survival-duration surrogates • Filling gaps in the QALY model • Utility over health profiles • Example decision analysis • Proportionate-duration surrogates • Open issues
QALY Model • QALYs are the most important and broadly used method for evaluating health quality. • Panel on Cost Effectiveness in Health and Medicine (Gold et al. 1996): Medical CE studies should incorporate morbidity and mortality consequences into a single measure using QALYs.
Problems with QALYs • Numerous studies have demonstrated that the correlation between one’s current health and the time-tradeoff or standard gamble utility for that health state is at best modest. (Tsevat 2000) • Willingness to trade away time often much less than one might expect. • Miyamota and Eraker (1988): Subjects might accept a tradeoff of life duration for improved health quality when remaining lifetime was long, but decline such tradeoffs if remaining lifetime was short. • This behavior cannot be accommodated within the QALY model.
Problems with QALYs (cont.) • Maximum endurable time: Subjects can tolerate no more than a particular time in an undesirable health state, beyond which each additional increment of time decreases overall utility. • Miyamoto et al (1998) report a patient who regarded his health state as almost intolerable, but who wanted to live at least 5 more years to see his son graduate from high school. • Sutherland et al (1982): 6-9/20 MET preferences among physicians and scientists, depending on health state evaluated. • Stalmeier et al (2001) report: • > 50% MET preferences for low QALY health states among students; • 10/14 MET preferences among migraine patients • 12/27 MET preferences among esophagectomy patients • Such behavior cannot be accommodated within the QALY model.
Outline of talk • QALYs/ Problems with QALYs • Health quality versus life quality: Extrinsic goals • Revising the QALY assumptions • Survival-duration surrogates • Filling gaps in the QALY model • Utility over health profiles • Example decision analysis • Proportionate-duration surrogates • Open issues
Health Quality vs. Life Quality • Hypothesis (Tsevat): QALYs capture quality of health, but not quality of life. • Goals related to quality of health tend to be ongoing – their impact is modulated by duration • increase mobility • eliminate pain • reduce emotional stress.
Health Quality vs. Life Quality (cont.) • Goals related to quality of life may be extrinsic – their impact is not modulated by duration: • an author might want to complete a book; • a politician might strive to achieve higher office; • an engineer or architect might endeavor to see a project to completion; • many individuals seek to have children and raise families.
Health Quality vs. Life Quality (cont.) • Schwartz et al (2006): • Community Study • Random-digit dialing telephone interviews • 50 Chicago-area residents • Patient Study • In-person interviews • 100 inpatients (University of Illinois Hospital, Jesse Brown VA Hospital) • In each study, participants provided up to five goals (three 5-year goals, one 10-year goal, one life goal)
Health Quality vs. Life Quality (cont.) • Schwartz et al (2006):Taxonomy of reported goals
Representative Goals by Category(Schwartz et al 2006) • Education: “finish college”, “go back to school” • Family • Self: “Get married”, “Have children” • Family member: “See daughter finish high school”, “See son get married” • Health and Fitness: “lose weight”, “complete marathon” • Personal Fulfillment: “spend more time in charitable activity”, “write a book” • Professional • Job: “get a job”, “own a business” • Retirement: “retire” • Travel: “travel to Europe”, “travel” • Wealth • Real Property: “buy a house”, “invest in property” • Personal Property: “buy a new car”, “own a boat” • Financial Security “become financially secure”, “win the lottery”
QALY model and Extrinsic Goals • In the QALY model, quality of health is given weight proportional to health duration. • It follows that the QALY model cannot directly account for extrinsic goals, whose importance is by definition independent of duration.
Outline of talk • QALYs/ Problems with QALYs • Health quality versus life quality: Extrinsic goals • Revising the QALY assumptions • Survival-duration surrogates • Filling gaps in the QALY model • Utility over health profiles • Example decision analysis • Proportionate-duration surrogates • Open issues
Assumptions underlying the QALY Model • Assumptions on preferences yielding the QALY form: Pliskin et al. (1980), Miyamoto et al. (1998), and Miyamoto (1999). • Preference model: • Quality/life duration pairs (q,t). • Theorem (Miyamoto et al 1998): A1 & A2 U(q,t) = UQ(q)UT(t) (Generalized QALY model)
Assumptions underlying the QALY model (Miyamoto et al 1998) Quality/life duration pairs (q,t). A1. The zero condition: Preferences between states of health disappear when survival duration is zero, that is, for all states q, q of health, (q,0) ~ (q,0). A2. Generalized utility independence (GUI) for lifetime (Standard gamble independence). Any two conditional preference relations over lifetime gambles, given health states q and q not equivalent to death, are either identical or reversed.
Failure of the zero condition for extrinsic goals • Goal achievement/ Quality/ Life duration triples (g,q,t) • Goal achievement may be preferred to non-achievement even if life duration is zero: (g = Achieved, q, t = 0) (g = Not achieved, q, t = 0)
Revised assumptions allowing for extrinsic goals Goal/ quality/ life-duration triples (g,q,t). B1. Conditional zero condition: For each level g of extrinsic goal achievement, preferences for health quality disappear when life duration is zero, that is, for all health states q, q, (g,q,0) ~ (g, q,0). B2. Generalized utility independence (GUI) for lifetime. Any two conditional preference relations over lifetime gambles, given health states q and q not equivalent to death, and goal achievement levels g and g, are either identical or reversed. B3. Additive independence of extrinsic goal attainment and health quality given life duration.
Revised assumptions allowing for extrinsic goals Goal / quality / life-duration triples (g,q,t). Theorem (Hazen 2003): B1+B2+B3 are equivalent to U(g,q,t) = UQ(q)UT(t) + kGUG(g).
Utility function incorporating extrinsic goals The utility model: U(g,q,t) = UQ(q)UT(t) + kGUG(g) Interpretation: UQ(q)UT(t) QALYs UG(g) Utility for goal achievement level g kG Tradeoff weight for goal achievement
Outline of talk • QALYs/ Problems with QALYs • Health quality versus life quality: Extrinsic goals • Revising the QALY assumptions • Survival-duration surrogates • Filling gaps in the QALY model • Utility over health profiles • Example decision analysis • Proportionate-duration surrogates • Open issues
Survival-duration surrogate for extrinsic goal achievement • Achievement of an extrinsic goal may require time commitment – say estimated time commitment is tG. • Simple and convenient surrogate for goal achievement: Whether survival duration t exceeds tG. Only two levels {0,1} of goal achievement Can take UG(g) = g.
Interpreting kGwhen there is a survival-duration surrogate Assumptions UG(g) = g (survival duration surrogate) UT(t) = t (no discounting) Therefore U(g,q,t) = UQ(q)t + kG [t ≥ tG]. Assessment question: What quality-of-life decrement q* q would you be just willing to accept to increase survival duration from just below tGto just above tG?
Interpreting kGwhen there is a survival-duration surrogate (cont) • Therefore (g=0, q*, tG-) ~ (g=1,q, tG+) U(g=0, q*, tG-) = U(g=1,q, tG+) 1tG + kG0 = UQ(q) tG + kG1 • Solve to obtain kG / tG = 1 – UQ(q). • Conclusion: kG / tG is the quality of life increment that one would be just willing to sacrifice to increase survival from slightly belowtGto slightly abovetG.
Outline of talk • QALYs/ Problems with QALYs • Health quality versus life quality: Extrinsic goals • Revising the QALY assumptions • Survival-duration surrogates • Filling gaps in the QALY model • Utility over health profiles • Example decision analysis • Proportionate-duration surrogates • Open issues
Goal model allows max endurable time Health profile h: Survive for duration t in undesirable health state with utility uQ < 0. U = uQt + kG[t ≥ tG] Utility decreases until t exceeds tG, where time goal is achieved.
Max endurable time as usually portrayed Stalmeier, Busschbach, Lamers, Krabbe, Health Econ (in press) Stalmeier, Chapman, de Boer, Lanschot , Tech Assessment in Health Care (2001)
Max endurable time as usually portrayed U = uQt + kG[t ≥ tG] • Assume tG is uncertain with distribution FG. • Then E[U] = uQt + kGFG(t) • Resulting graphs of utility vs. life duration conform to usual portrayal.
Goal model allows tradeoff reluctance • If reduction in survival time interferes with goal achievement, then it may make sense not to trade away time for health improvement. uQ = 0.30, tG = 5 yr U = uQt + kG[t ≥ tG] kG = 0 (QALY model) kG = 4 yr
Goal model allows reluctance to gamble • Risks of death may be declined to the extent they interfere with goal achievement. uQ = 0.30, tG = 5 yr U = uQt + kG[t ≥ tG] kG = 0 (QALY model) kG = 4 yr
Outline of talk • QALYs/ Problems with QALYs • Health quality versus life quality: Extrinsic goals • Revising the QALY assumptions • Survival-duration surrogates • Filling gaps in the QALY model • Utility over health profiles • Example decision analysis • Proportionate-duration surrogates • Open issues
Extending to utility over health profiles • Health profile h: A function which assigns health state q = h(s) to every time instant s in some interval [0, th]. • The informal approach for QALYs (Pliskin et al 1980): • Assumption Q1: For any health profile h there is a level q = Q(h) of health quality such that h ~ (q,th). • Assumption Q2: Q(h) satisfies the time-weighted average equation • Conclusion:
Extending to utility over health profiles • The informal approach for QALYs (Pliskin et al 1980), with no time discounting: • Assumption Q1: For any health profile h there is a level q = Q(h) of health quality such that h ~ (q,th). • UT(t) = t • Assumption Q2: Q(h) satisfies the time-weighted average equation • Conclusion: Sum the QALYs along the path
Extending to utility over health profiles and extrinsic goals • Extrinsic goal achievement is not time-modulated, so does not accrue over time, but instead is associated holistically with the entire life profile of an individual. • For modeling purposes, then, we consider preferences over pairs (g,h), where h is a health profile and g is a level of extrinsic goal achievement. • Assumption Q1 extended: For any health profile h and goal achievement level g, there is a level q = Q(h) of health quality such that (g,h) ~ (g,q,th). • Conclusion (under no time discounting):
Extending to utility over health profiles and extrinsic goals • Note: Q(h) is assumed to not depend on g. • Reasonable because the additive form U(g,q,t) = UQ(q)UT(t) + kGUG(g) implies q,t utility independent of g, so why not h utility independent of g?
Outline of talk • QALYs/ Problems with QALYs • Health quality versus life quality: Extrinsic goals • Revising the QALY assumptions • Survival-duration surrogates • Filling gaps in the QALY model • Utility over health profiles • Example decision analysis • Proportionate-duration surrogates • Open issues
Example Decision Analysis • Decision to undergo carotid endarterectomy – a Markov chain analysis performed by Matchar & Pauker (1986).
Example Decision Analysis • We add an extrinsic goal represented by survival-duration surrogate tG = 6 yr. • We take goal weight kG= 1.2 yr. (Willing to decrease health quality by kG/tG = 0.20 in order to increase survival duration from just below the 6-year survival goal to just above it.)
Example Decision Analysis Results tG = 6 years, kG = 1.2 years
Outline of talk • QALYs/ Problems with QALYs • Health quality versus life quality: Extrinsic goals • Revising the QALY assumptions • Survival-duration surrogates • Filling gaps in the QALY model • Utility over health profiles • Example decision analysis • Proportionate-duration surrogates • Open issues
Partial goal achievement • Proportionate-duration surrogate for degree of goal achievement: g = min {1,t / tG} = survival time as a percentage up to 100% of a critical duration tG. • U(g,q,t) = UQ(q)UT(t) + kGmin{1,t/tG}
Proportionate-duration max endurable time preference • tG = 5 yr, kG = 4 yr
Proportionate-duration willingness to trade off for full health • tG = 5 yr, uQ = 0.3 • kG = 0 (QALY model) • kG = 4 yr
Proportionate-duration risk of death willing to accept for full health • tG = 5 yr, uQ = 0.3 • kG = 0 (QALY model) • kG = 4 yr
Proportionate-duration utility and the QALY model • Proportionate-duration utility w/o discounting • This is equivalent to: • UQ(q) + kG/tGQALYs per unit time up to time tG • UQ(q) QALYs per unit time after time tG • This is a modified QALY model
Proportionate-duration utility and the QALY model • Theorem: Suppose degree of extrinsic goal achievement is measured by the proportionate-duration surrogate, and there is no time discounting. Then the utility of a health profile h is equivalent to the QALY of a modified health profile hG in which all health states q occupied before time tG are replaced by states q+ having health quality UQ(q+) = UQ(q) + kG/tG. • Implication: Standard software can be used to compute extrinsic-goal utility with a proportionate-duration surrogate goal.
Outline of talk • QALYs/ Problems with QALYs • Health quality versus life quality: Extrinsic goals • Revising the QALY assumptions • Survival-duration surrogates • Filling gaps in the QALY model • Utility over health profiles • Example decision analysis • Proportionate-duration surrogates • Open issues
Open issues • Multiple simultaneous goals • Future goals • Once current goal(s) are achieved, future goals are likely to arise. Should this be modeled? If so, how? • Note that no one asks this kind of question for QALYs - ongoing goals represented by QALYs are assumed never to change.
Open issues • Population issues • Heterogeneous goals across a population – how to account for these? • Heterogeneous parameters kG, tG – how to account for these? • Note for QALYs, all that matters is the population average QALY for each health state, so heterogeneity issues are not as significant for the QALY model.
Conclusion • Utility functions that include an extrinsic goal component • can account for observed violations of the QALY model (maximum endurable time preference, reluctance to trade off time for quality) • can do so prescriptively, thereby providing a coherent basis for including such goals in decision and cost-effectiveness analyses.