The economist Amartya Sen has said that world development ought to focus on the capabilities that people achieve, and not simply on aggregate economic indicators. In that spirit, the United Nations since 1990 has published several Human Development (HD) reports that assess nations in terms of health and education conditions, as well as income or wealth. Many countries have assessed their own HD. In The Measure of America, Sarah Burd-Sharps, Kristen Lewis, and Eduardo Borges Martins apply this approach to the United States. They describe variation in health, education, and income conditions across American states, congressional districts, and social groups.