The Human Development Report Office of UNDP calculates the annual Human Development Index (HDI) and uses it to rank countries into four tiers of human development. HDI is a summary measure of achievements in three key dimensions of human development: a long and healthy life, access to knowledge and a decent standard of living. The HDI is the geometric mean of normalized indices for each of the three dimensions. Data sources for the HDI include Life expectancy at birth from UNDESA, Expected years of schooling from the UNESCO Institute for Statistics, United Nations Children’s Fund (UNICEF) Multiple Indicator Cluster Surveys and ICF Macro Demographic and Health Surveys, Mean years of schooling from Barro and Lee, UNESCO Institute for Statistics, Human Development Report Office updates based on UNESCO Institute for Statistics, UNICEF Multiple Indicator Cluster Surveys and ICF Macro Demographic and Health Surveys, and GNI per capita from IMF, UNSD and World Bank.