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3.1 DALY versus QALY
DALYs and QALYs are technically similar in that they both express health in time (life years) and give a weight to years lived with a disease. In the terminology of Gold et al. ( 2002) , both measures are HALYs (Health adjusted life Years). Gold et al. give a comprehensive review of the differences between DALYs and QALYs. DALYs measure health loss and QALYs health gain so they express an inverse value. That problem is overcome by looking at differences (in absolute terms) between scenarios or interventions. More importantly the measures originate from different disciplines, which causes the disease weights to be measured in a different way with a different interpretation, resulting in different values. There is debate about the use of QALYs vs DALYs (Sassi, 2006) as they give different outcomes, but a large part of the differences is explained by whether or not age weighting and discounting is applied. The original DALY formulation (Murray, 1994) incorporates age-weighting and discounting. In practise, the difference between a DALY and a QALY depends on whether the quality of life is expressed as a loss (DALY) or a gain (QALY), as is illustrated by the weight for death (1 for DALY, 0 for QALY). Additional differences are caused by the way disease weights are measured. Given the variability in disease weights and the influence of age weighting and discounting it seems that the choice of the common currency should be determined by the choice for disease weights and whether or not to apply age weighting and discounting (but note that specific functionality for age weighting and discounting is not provided in the current version of the Qalibra software).