The Human Development Index is the geometric mean of three dimension indices built from four indicators
Background
The Human Development Index is a summary measure of average achievement in key dimensions of human development: a long and healthy life, being knowledgeable, and having a decent standard of living.
The Index is the geometric mean of normalized indices for each of the three dimensions.
The health dimension is assessed by life expectancy at birth.
The education dimension is measured by mean years of schooling for adults aged 25 years and more, together with expected years of schooling for children of school entering age.
The standard of living dimension is measured by gross national income per capita, and the Index uses the logarithm of income to reflect the diminishing importance of income as gross national income rises.
Three dimensions are therefore captured by four indicators: the two education indicators are averaged first, and the three dimension indices are then combined as a geometric mean.
The Index is published by the Human Development Report Office of the United Nations Development Programme.
It can be used to question national policy choices by asking how two countries with the same level of gross national income per capita can end up with different human development outcomes.
In plain words
Geometric mean The nth root of the product of n numbers — Using it means a very low score in one dimension cannot be hidden by a high score in another.
Expected years of schooling The schooling a child of school entering age can expect to receive — It is one of the two education indicators, the other being mean years of schooling for adults aged 25 and over.