Disability Adjusted Life Years (DALYs)

DALYs is a measure for the overall ‘burden of disease’. Originally developed by the World Health Organization (WHO), it is being increasingly used in the literature on public health. It is designed to quantify the impact of premature death and disability on a population by combining them into a single measure. Traditionally, health liabilities were expressed using one measure: expected or average number of ‘years of life lost’ (YLL). This measure does not take the impact of disability into account, which can be expressed as years lived with disability (YLD). DALYs are calculated by taking the sum of these two components: i.e., years of life lost plus years lost to disability or DALY = YLL + YLD.

The basic method of computing the DALYs is as follows. Suppose N is the number of deaths in a certain population and L is the standard life expectancy at age of death in years, then YLL = N * L (where the symbol * stands for multiplication). Because YLL measures the incident on the stream of lost years of life due to deaths, an incidence perspective is also taken for the calculation of YLD. To estimate YLD for a particular cause in a particular time period, the number of incident cases in that period is multiplied by the average duration of the disease and a weight factor that reflects the severity of the disease on a scale from 0 (perfect health) to 1 (dead). If ‘I’ is the number of incident cases, DW the disability weight and L the average duration of the case until remission or death (in years), then YLD = I * DW * L.

Looking at the burden of disease in terms of DALYs can reveal surprising things about a population’s health. For example, a 1990 WHO report indicated that 5 of the 10 leading causes of disability were psychiatric conditions. Psychiatric and neurologic conditions account for 28 per cent of all years lived with disability. Thus, psychiatric disorders, while generally not seen as a major epidemiological problem, are revealed by consideration of disability years to have a huge impact on populations.

Both the methods, QALY and DALY, are critiqued for their drawbacks. It is pointed out that studies using QALY as an indicator of health measures the benefits from a health programme in terms of the increment in health status over the period for which the intervention was effective. Thus, the QALY methodology inherently assumes that the entire change in the health status of an individual, before and after the implementation of the programme, can be attributed solely to the programme in question. However, non medical factors like income and education also contribute to the effect (i.e. impact on health) of a programme. Decisions regarding the inter-sectoral allocation of resources, which make the resources available to one sector at the cost of the other, are important from the policy angle. On the one hand, there is a possibility of improvement in the health status of individuals owing to increase in public expenditure on health programmes. On the other hand, such a policy choice will have an adverse effect on health owing to reduction in the allocation of funds for education, sanitation, etc. The measures, QALY and DALY, by disregarding the influence of such factors do not help in analysing the effectiveness of such a policy choice.