The Human Development Index (HDI) is a composite statistic used to measure development. By integrating life expectancy, education attainment, and income indicators, countries are ranked into four tiers of human development. The education component is comprised of two indicators: expected years of schooling and mean years of schooling. The income part is calculated by GNI per capita (PPP$). The index has been continually increasing the number of countries it covers in its report; in 2013, the index included 187 countries.