We will wrap up on measures of disease burden with two measures that are intense in their calculations and powerful in their meaning. First, disability adjusted life years or DALYs. DALYs were initially developed for the World Bank's, World Development Report in 1993. This particular disease burden measure is used in prioritization and decision making and is one of the most widely used measures of disease burden. DALYs can be thought of as an extension of the years of potential life lost or YPLLs. DALYs are a measure of the healthy life lost by virtue of being in states of poor health or disability. The estimation of DALYs requires information on mortality but also on non fatal health outcomes. In an ideal situation, everyone would live to the age of life expectancy in perfect health. But we know that is not reality. A DALY is a measure of the gap between that ideal situation and the reality of poor health and disability. You will see DALYs stratified by gender, due to the differences in life expectancy by gender, more specifically, the fact that women have longer life expectancies as compared to men in most cultures. Because DALYs are the number of lost healthy years, the public health goal is to reduce DALYs. DALYs can be used to measure trends and the severity of illness and often DALYs are measured only at the national level. You will understand why, once we dig a little deeper into how DALYs are estimated. The conceptual calculation for DALYs is adding the years of life lost due to premature mortality with the years lived with disability. The first part is straightforward to estimate. The years of life lost due to premature mortality can be estimated by multiplying the number of deaths with the standard life expectancy. The second part is a little more challenging. The years lived with a disability is calculated by multiplying the number of incident cases of disease with the disability rate and the average duration of disability. As you are likely already thinking about, these are hard numbers to find. The instant number of cases of a specific disease or disability will need to be informed by either public health information systems or other studies specifically designed to estimate incidents. The disability rates are agreed upon by experts in DALY estimation. And finally the average duration of disability is a very challenging estimate that often takes unique data sources to estimate. When calculating DALYs you must calculate DALYs for each cause as you can see in this slide. So you can see how this is continuing to get more computationally complex. But the concepts behind the calculations make sense. Measuring the years of healthy life lost by combining the years of life lost due to premature mortality with the years lived with the disability, provides a powerful measure of disability and a loss of healthy life. Here's the list of disability weights. These weights are decided upon by experts within the disabilities, as well as experts in the computation of DALYs. Notice, these weights are stratified. There's the mean disability weight of untreated disease and there's the mean disability weight for the treated form of the disease. Although calculating DALYs is out of the scope of the technical capacity of this class, the information depicted in this figure, is interesting and I hope at this point, you can interpret it. Looking at the WHO classification of countries by income and geography, we see that in Africa, it is the years of life lost due to premature mortality that result in such a high estimate of DALYs per 1000 population. This is in contrast to high income countries, the Americas, and the Western Pacific, where both the years of life lost due to premature mortality and the years lived with a disability contribute about equally to the overall disease burden described by DALYs. DALYs inform public policy and program because they are insightful measure of the burden in the population due to premature mortality and disability. Let's take a break.