It is also important to understand how Years Lived with Disability (YLD) is defined, by whom, and at what time in the disease course [4]. First, disability represents an “objective alteration of behavior or performance at the individual level”. Disability falls between impairment, which is defined by symptoms at an organ level, and handicap, which is defined by changed interactions with others at the social or environmental level due to disability. To illustrate, if an individual suffers from a stroke, s/he might have symptoms of unilateral arm and leg weakness (impairment), which limits her/his ability to walk independently (disability) and her/his ability to work in a job that requires walking (handicap). Second, researchers have typically surveyed medical professionals and public health experts to rank symptom states to quantitatively estimate YLD for myriad disease states. Other individuals, including but not limited to patients, families, caregivers, general public, insurance companies, and legal experts, might offer complementary perspectives on how YLDs should be estimated yet have not been incorporated in these estimates to date. Third, YLD estimates can be sensitive to the time course of the disease, particularly for non-communicable, chronic diseases, which can have long periods of minimal to no symptoms followed by acute shocks and gradual recovery to or near baseline. In the stroke example, the immediate post-stroke disability can be substantially different than 3, 6, or 12 months later and can be dependent upon access to rehabilitation and medical therapy. As such, YLD estimates may be susceptible to reporting bias by experts based on their previous clinical or health experiences.

Newer estimates of disease burden incorporate costs and financial risk through measures such as catastrophic health spending (based on the proportion of health spending relative to non-food expenditures) and distress financing (based on risky financial activities to pay for health, including borrowing money or selling assets) [5]. These complementary measures of financial protection, or lack thereof, are associated with individuals and families falling into poverty. Because health systems are evaluated in terms of quality, access, and financial protection [6], these measures of disease burden will likely gain more attention.

Using the International Classification of Diseases framework, the Global Burden of Disease project employed systematic searched published and unpublished data on causes of death through a variety of sources, including the World Health Organization mortality database, national vital registration systems, verbal autopsy-based sample registration systems, demographic surveillance systems, cancer registers, crime reports, mortuary data, among others [3]. In the case of so-called “garbage codes” that have been deemed implausible causes of death, available data were used to reclassify the causes of death [1]. The project team then incorporated these best available data into advanced, multi-level statistical models and imputation methods to estimate the causes of death among all countries from 1980 through 2010.

The Global Burden of Disease project is not without its critics who express concern about the complex analytic methods and frequent use of imputation to estimate data for countries that do not have accurate, updated mortality data. Some fear that the Global Burden of Disease, which is largely funded by the Bill & Melinda Gates Foundation, a private non-governmental organization, may lead to reduced public investments in vital registration systems, a basic public health function that currently covers less than half of the world’s population [7]. Nevertheless, the Global Burden of Disease project represents the most comprehensive and accessible summary of contemporary global disease burden, including providing estimates for non-communicable, chronic diseases.