At the time of the 2003 review of the list, the following three criteria were used by the UN: low income, in the light of a three-year average estimate of the gross national income per capita (under $750 for cases of addition to the list, above $900 for cases of graduation); weak human assets, as measured through a composite Human Assets Index; and economic vulnerability, as measured through a composite Economic Vulnerability Index.

http://unctad.org/en/Pages/ALDC/Least%20Developed%20Countries/The-Least-Developed-Countries-Report.aspx UNCTAD’s Least Developed Countries Report.