The Human Development Index (HDI) is a composite index measuring average achievements in three key dimensions of human development: (1) long and healthy life (life expectancy), (2) knowledge (education), and (3) decent standard of living (GNI per capita). It is normalised on a scale from 0 (lowest) to 1 (highest). Countries are classified as very high, high, medium, or low human development based on their HDI score.