It is useful to briefly review the major data sources available for the GBD 2013 study and other estimates of child morbidity and mortality. Globally, only about half of all births are registered and fewer than half of all deaths are actually captured by well-functioning civil registration and vital statistics.4 As in most high-mortality, sub-Saharan countries, Nigeria, the Democratic Republic of the Congo, and Angola record fewer than 10% of deaths in formal systems and data on stillbirths are essentially missing. The GBD 2013 initiative must therefore rely on a series of alternative sources of data for countries of greatest concern. Among the most important of these are the Demographic and Health Surveys, which are nationally representative household surveys conducted intermittently in a variety of countries.5 Also, UNICEF implements Multiple Indicator Cluster Surveys, which generate data on the health and well-being of women and children in many low- and middle-income areas of the world.6 While extremely useful, these surveys are not conducted annually and may leave large temporal gaps in national or regional trend data. For example, in Guatemala, a country with relatively high levels of child malnutrition and mortality, no Demographic and Health Survey was conducted between 1999 and 2015 and no Multiple Indicator Cluster Survey has yet been performed. In addition, these surveys rarely provide accurate subnational estimates, a weakness of some importance in large countries that are plagued by regional conflicts or traditional inequalities, such as Nigeria or the Democratic Republic of the Congo. Published data sources are also used, including verbal autopsy series, which conduct detailed assessments of the circumstances of selected deaths; sibling histories, which are useful for documenting mortality among adolescent and adult populations; and diverse studies that include some form of mortality data. Although crucial sources of data, these studies can be highly heterogeneous in both purview and methods. The GBD 2013 study investigators are, of course, acutely aware of the problems generated by the lack of accurate source data. Indeed, the GBD 2013 study’s methodological innovations have been developed precisely to address this issue.