Mortality Data from the National Vital Statistics SystemMortality data from the National Vital Statistics System (NVSS) are a fundamental source of demographic, geographic, and cause-of-death information. This is one of the few sources of health-related data that are comparable for small geographic areas and are available for a long time period in the United States. The data are also used to present the characteristics of those dying in the United States, to determine life expectancy, and to compare mortality trends with other countries.