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7. Calculation of DALYs for continuous health effects
The treatment of a continuous effect will depend on whether its severity as a disease (as measured by a DALY weight) is a step function, applying only above or below some critical effect size. If so, then it may be simpler to model the effect as quantal, e.g.. as a dose-response relation for exceeding the critical effect size, and calculate the DALY loss using Equation (1) above.
If the severity of the disease is a continuous or multi-step function of the effect size, then it will be necessary to use Equation (2):
Where the new terms are:
effect = magnitude of health effect
w ( effect ) = DALY weight expressed as a function of effect magnitude.
When assessing the risks and benefit of a dietary change, effect will be related by a dose-response function to the intake of some contaminant or nutrient. It is important to note that the current version of the Qalibra software assumes the response is the same for all individuals at a given dose . It does not allow variation around the dose-response relationship to be treated as real variation in the response of individuals at the same dose (it does allow this variation to be treated as uncertainty, see later). An alternative equation, which allows for (and integrates over) variation in response between individuals at the same dose, is given by Hoekstra et al. (submitted). The potential impact of this limitation of the Qalibra software should be taken into account when considering unquantified uncertainties affecting the assessment (see later).