The Human Development Index (HDI) is a summary measure of average achievement in key dimensions of human development: a long and healthy life, being knowledgeable and have a decent standard of living. The HDI is the geometric mean of normalized indices for each of the three dimensions. The underlying indicators are life expectancy at birth, mean years of schooling (age 25+), expected years of schooling (children entering school) and GNI per capita (PPP $).