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. Currently, Seychelles is the only African country that falls into the very high human development category. Somalia has the lowest HDI in both Africa and the world according to the list.