This index is indicative of the challenge in a country to meet the right to education (Universal Declaration of Human Rights Article 26), and the right to information and expression (Article 19) in the mother tongue (See also International Mother Language Day, 21 February).
A value between 0 and 1; 1 means total diversity, i.e. no two inhabitants of the country meeting at random would have the same mother tongue.
The index is close to one for Papua New Guinea (where 830 languages are spoken by about 6 million inhabitants).
For information on other countries, see Language diversity around the world (Daily Chart, The Economist, Feb. 15, 2012).