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Testing the Additive Independence Assumption in the QALY Model

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  • Anne Spencer

    (Queen Mary, University of London)

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    Abstract

    Quality-Adjusted Life Years (QALYs) estimate the utility derived from health states by taking account of life expectancy and quality of life. In applying QALYs to situations where health varies over time, it is usual to assume that we can add the utilities from constituent health states. This assumption of additive independence has been challenged by research suggesting that people's preferences for health states are affected by the sequence in which health states occur. This paper investigates two tests of additive independence as well as a test of the consistency of preferences over time. The main test of additive independence is based on a new method using stylised health profiles of deteriorating, improving or temporarily-improving health. The advantage of this new method is that the test relies only on a comparison of health states occurring in the same time period and therefore controls for the effects of time preference. The other test of additive independence determines whether people strictly prefer one type of health profile over another and collects qualitative data on the issues considered by people in these choices. In the main test of additive independence, only one of the two cases considered detected a statistical difference. In the other test of additive independence, the sample was split almost equally between those strictly preferring one or other of the choices. The qualitative data revealed contrasting viewpoints about the benefits derived from such choices which led to this split. The paper also found that preferences were consistent over time. The tests of additive independence were, therefore, unable to conclusively reject additive independence.

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    File URL: http://www.econ.qmul.ac.uk/papers/doc/wp427.pdf
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    Bibliographic Info

    Paper provided by Queen Mary, University of London, School of Economics and Finance in its series Working Papers with number 427.

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    Date of creation: Nov 2000
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    Handle: RePEc:qmw:qmwecw:wp427

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    Keywords: Utility measurement; Additive independence; Health profiles;

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    1. Robinson, Angela & Dolan, Paul & Williams, Alan, 1997. "Valuing health status using VAS and TTO: What lies behind the numbers?," Social Science & Medicine, Elsevier, vol. 45(8), pages 1289-1297, October.
    2. Bleichrodt, Han, 1995. "QALYs and HYEs: Under what conditions are they equivalent?," Journal of Health Economics, Elsevier, vol. 14(1), pages 17-37, May.
    3. Curtis, Sarah & Gesler, Wil & Smith, Glenn & Washburn, Sarah, 2000. "Approaches to sampling and case selection in qualitative research: examples in the geography of health," Social Science & Medicine, Elsevier, vol. 50(7-8), pages 1001-1014, April.
    4. Johnston, Katharine & Brown, Jackie & Gerard, Karen & O'Hanlon, Moira & Morton, Alison, 1998. "Valuing temporary and chronic health states associated with breast screening," Social Science & Medicine, Elsevier, vol. 47(2), pages 213-222, July.
    5. Paul Dolan & Claire Gudex, 1995. "Time preference, duration and health state valuations," Health Economics, John Wiley & Sons, Ltd., vol. 4(4), pages 289-299, 07.
    6. Torrance, George W., 1986. "Measurement of health state utilities for economic appraisal : A review," Journal of Health Economics, Elsevier, vol. 5(1), pages 1-30, March.
    7. Dolan, Paul & Cookson, Richard, 2000. "A qualitative study of the extent to which health gain matters when choosing between groups of patients," Health Policy, Elsevier, vol. 51(1), pages 19-30, February.
    8. Loomes, Graham & McKenzie, Lynda, 1989. "The use of QALYs in health care decision making," Social Science & Medicine, Elsevier, vol. 28(4), pages 299-308, January.
    9. Charmaz, Kathy, 1990. "'Discovering' chronic illness: Using grounded theory," Social Science & Medicine, Elsevier, vol. 30(11), pages 1161-1172, January.
    10. Ann M. Holmes, 1998. "Measurement of Short Term Health Effects in Economic Evaluations," PharmacoEconomics, Springer Healthcare | Adis, vol. 13(2), pages 171-174.
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