Moving on to Years of Potential Life Lost or YPLL, the next measure of disease burden that we're going to learn about. JRR Tolkien said, not all those who wander are lost. Tolkien is the author of high fantasy classic literature, including The Hobbit and The Lord of the Rings. I like this quote for the discussion of years of potential life lost because it reminds me that, although the YPLL measure is very straightforward, it is dramatically impacted by deaths at young ages. So there are efforts to reduce that impact by discounting the years of life that are lost. More on that in a few minutes. YPLL measures premature mortality and is heavily influenced by deaths at younger ages. Oftentimes, you will see YPLL estimates presented with mortality rates. For example, you may have seen the leading causes of death estimated using age adjusted mortality rates and YPLLs. YPLLs can be measured if you have individual level data, meaning you have data about individuals who have died. If you don't have access to individual level data, but rather have information about deaths aggregated by age at death, YPLLs can also be calculated. The basic concept is that YPLLs are a summation of the years between death and an endpoint age that usually approximates life expectancy. The age of 75 is often used by the US National Center for Health Statistics as an approximation of current life expectancy. YPLLs are often convenient to calculate and can measure trends over time. If you stratify they YPLL by age group, YPLLs can be used to describe severity of illness with greater severity oftentimes resulting in more deaths at younger ages and higher YPLLs. Data on deaths are needed to estimate YPLLs. So they can be measured at the local, state, or national level, assuming data on deaths are available. A quick demonstration for calculating YPLLs on individual level data. Each row in this table is one person who died. The age at death is noted in the second column. The chosen end age point of 75 is located in the third column. The years lost are estimated as the end point age minus the age at death. Those who died at ages older than 75 have 0 imputed for their years lost. The years lost are added up for a total YPLL of 191 years in this population of ten individuals. To calculate YPLLs using group level data, you find the midpoint of the age group at death. There's a number line above this table to demonstrate that the midpoint of the first age group, which is 0.0 to 14.9 years, is 7.5 years. A midpoint is a basic mathematical concept. But when it becomes tricky, it's good to use a number line. The midpoint of the age group at death is in the second column. The number of deaths in each age group is in the third column. The selected end age is in the fourth column. Years lost within each age group is calculated by subtracting the midpoint from the end age and multiplying by the number of deaths. Similar to the individual data approach, the years lost are then added up to estimate the total YPLL. If you have access to the total number of people in the population, you can also calculate the YPLL rate per 1,000 population by dividing the total YPLL by the total population and multiplying by 1,000. The YPLL measure is particularly sensitive to deaths occurring at younger ages. Sometimes it's too sensitive, so a discounting strategy is used. A discounting strategy is based on the idea that the productive years of life are more valuable than the very young and the very old years. There is also a convention that YPLL should be discounted at an annual rate of 3%. The idea of discounting is all an attempt to lessen the impact of deaths at younger ages. For a detailed discussion of discounting YPLL, see the resource listed here. I'd like to show you to quick examples of YPLLs. Here are the years of potential life lost per 100,000 in Baltimore City and the State of Maryland. Baltimore City is in the dark line with the closed circles on top, and the lower line represents the years of potential life lost per 100,000 in the State of Maryland. You will notice that the data presented here are YPLLs per 100,000 population. It's a rate. And that the YPLL rate for Baltimore is much higher than for the State of Maryland. And in this example of YPLL, the YPLLs are broken into the top ten causes of death. Cost specific YPLL estimates are one way to look at the severity of an illness. The higher the YPLLs, the more the severe the disease. This concludes our brief section on YPLLs.