1 / 49

Evaluation of Medicine

Evaluation of Medicine. Two types: Societal level Economic evaluation Individual level medical decision making. Economic Evaluation. Comparison of costs and benefits Central question: How can we express the benefits of health care numerically?. Approaches.

warner
Download Presentation

Evaluation of Medicine

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Evaluation of Medicine • Two types: • Societal level • Economic evaluation • Individual level • medical decision making

  2. Economic Evaluation • Comparison of costs and benefits • Central question: How can we express the benefits of health care numerically?

  3. Approaches • Ignore health benefits - Cost minimization • Express benefits as life-years gained • Contingent valuation - CBA • Express benefits as utilities - CUA

  4. Medical Decision Making • Optimal treatment selection • Common approach: perform decision analysis expressing benefits in utility terms

  5. QALYs • Question: which utility model? • Most common model: • Quality-Adjusted Life-Years (QALYs)

  6. QALYs • Additive model: • Let (q1,..., qT) be health profile • QALY model: U (q1,..., qT) =  V(qi )

  7. Example • Living for 10 years with asthma • Suppose V(asthma) = 0.5 • QALYs (10 years asthma) = 5

  8. QALYs • Advantages • intuitively appealing • easy to use in practice • Disadvantages • may be too restrictive

  9. Two questions • QALY model: U (q1,..., qT) =  V(qi ) • How do we determine the utilities V(qi )? • Which are the assumptions underlying the QALY model?

  10. Basic Ingredients • Set of health states H • Set of lotteries P over H • Preference relation R over P • Representing function V:PIR such that V(P)  V(Q) iff x R y

  11. Assumptions • Health states are chronic: H = TQ • Health states are positive, i.e., preferred to death • Expected utility holds

  12. Expected Utility • V((Q1,T1), p, (Q2,T2)) = pU(Q1,T1) + (1p)U(Q2,T2) • U = a + bU, a real, b > 0

  13. Chronic Health States • Two attribute utility function U(Q,T) = V(Q)*T Q set of positive health states, T set of durations

  14. First characterization • Pliskin, Shepard & Weinstein (1980) • 3 conditions • (mutual) utility independence • constant proportional trade-off • risk neutrality wrt life-years

  15. Utility independence • Quality of life is utility independent of duration if preferences over lotteries on quality of life holding duration fixed do not depend on the level at which duration is held fixed • Duration is utility independent of quality of life if preferences over lotteries on duration holding quality of life fixed do not depend on the level at which quality of life is held fixed

  16. Formally • If (p1,(Q1,T);….;pm,(Qm,T)) R(r1,(Q1,T);….;rm,(Qm,T)) • then • (p1,(Q1,T);….;pm,(Qm,T)) R(r1,(Q1,T);….;rm,(Qm,T))

  17. And • If (p1,(Q,T1);….;pm,(Q,Tm)) R(r1,(Q,T1);….;rm,(Q,Tm)) • then • (p1,(Q,T1);….;pm,(Q,Tm)) R(r1,(Q,T1);….;rm,(Q,Tm))

  18. Standard gamble • (20y, Asthma)  ((20y, FH), 2/3, (20y, Death)) • Then also • (40y, Asthma)  ((40y, FH), 2/3, (40y, Death))

  19. Intermediate result • The following statements are equivalent: • utility independence holds • U is either additive, U(Q,T) = V(Q) + W(T), or multiplicative U(Q,T) = V(Q)*W(T)

  20. Hence • To arrive at the QALY model must (i) exclude the additive model and (ii) ensure linearity of W(T).

  21. Constant proportional tradeoffs • The preference relation satisfies constant proportional tradeoffs if (Q1,T1)  (Q2,T2) iff (Q1,T1)  (Q2, T2) for all Q1, Q2 in Q, T1, T2, T1, T2 in T and  nonnegative

  22. Time Trade-off • If (Q1,T1)  (Q2,T2) then • U(Q1) = T2/T1

  23. Example • (10 years asthma) ~ (8 years FH) • U(asthma) = 0.8 • (20 years asthma) ~ (16 years FH)

  24. Exercise • Show that CPT excludes the additive model • Hence • U(Q,T) = V(Q)*W(T)

  25. Risk neutrality • Risk neutrality wrt life-years • Risk neutrality for duration holds if for a fixed health status level all treatments with equal expected life duration are equivalent. • (20y., FH) ~ ((40y., FH), 0.5; (0y, FH))

  26. Implications • W(T) linear • Hence, have derived the QALY model

  27. Theorem • Under EU the following two statements are equivalent • The QALY model represents preferences for health • The preference relation satisfies utility independence, constant proportional tradeoffs and risk neutrality wrt life-years

  28. Less restrictive result • Take the opposite route • Start with risk neutrality wrt life-years • U(Q,T) is linear in life-years

  29. Hence • U(Q,T) = A(Q) + V(Q)*T • Note: negative health states are allowed

  30. Hence • Have to get rid of term A(Q) to obtain QALY model. • Assume zero condition: for duration zero all health states are equivalent

  31. Exercise • Show that the zero condition implies that A(Q) = 0 for all Q.

  32. Theorem • Under EU the following two statements are equivalent • The QALY model is representing • The preference relation satisfies risk neutrality wrt life-years and the zero condition

  33. Hence • In PSW representation can drop • utility independence • and can weaken CPT to zero condition

  34. Empirical evidence • Zero condition unobjectionable • People are risk averse wrt life-years • Hence, QALY model not descriptively valid • Normative status risk neutrality?

  35. More general model • U(Q,T) = V(Q)*W(T) • Miyamoto, Wakker, Bleichrodt & Peters (1998)

  36. Standard Gamble Invariance • For Q and Q unequal to death: (Q,T)  ((Q,Y), p, (Q,Z)) iff (Q,T)  ((Q,Y), p, (Q,Z))

  37. Then • U(Q,T) = V(Q)*W(T) + A(Q) • Zero condition: A(Q) = 0

  38. Theorem • Under EU the following two statements are equivalent • The nonlinear QALY model is representing • The preference relation satisfies standard gamble invariance and the zero condition

  39. One more result • Under EU the following two statements are equivalent • U(Q,T) = V(Q)*Tb • The preference relation satisfies standard gamble invariance and constant proportional trade-offs

  40. Empirical evidence There is support for utility independence Miyamoto&Eraker Bleichrodt&Johannesson Guerrero Bleichrodt&Pinto and constant proportional tradeoffs Bleichrodt&Johannesson

  41. However • Maximal endurable time • violates utility independence • Lexicographic preferences for low durations • violates constant proportional tradeoffs • more in line with increasing proportional tradeoffs

  42. Alternative measures • Mehrez & Gafni (1989): Healthy-years equivalent (HYEs) • (q1,..., qT)  (Full health, T´) • HYEs (q1,..., qT) = T´

  43. Claim • ´´HYEs impose no assumptions on the utility function and are therefore entirely general´´ • Q: Is this claim true?

  44. 2-stage measurement procedure • First stage: determine p such that (q1,..., qT) ((FH,T), p, death) • Second stage: determine T´ such that ((FH,T), p, death) (FH,T´)

  45. Argument • Two-stage gamble leads to same result as directly determining T´ from (q1,..., qT)  (Full health, T´) Questions: • Is this argument correct? • If so, is it true that HYEs are exactly as restrictive as QALYs?

More Related