The Human Development Index (HDI) is a summary measure of average achievement in key dimensions of human development: a long and healthy life, knowledge, and a decent standard of living. It is a standard means of measuring well-being. It is used to distinguish whether the country is a developed, developing, or underdeveloped country, and also to measure the impact of economic policies on quality of life. Countries fall into four broad categories based on their HDI: very high, high, medium, and low human development.