Focussing on ethnicity, they maintain a database of "unreached peoples" listed by country and language. As of 2009, they list 9,818 ethnic groups. These are further divided into 16,360 peoples-by-countries, counting national minorities individually, of which 6,641 are classified as "least reached". Ethnic groups are organized hierarchically in 251 "People Clusters" which in turn are divided in 16 "Affinity Blocks" (Arab World, East Asians, Eurasians, Horn of Africa-Cushitic, Iranian-Median, Jews, Latin-Caribbean Americans, Malay peoples, North American peoples, Pacific Islanders, South Asians, Southeast Asians, Sub-Saharan Africans, Tibetan / Himalayan peoples, Turkic peoples and Unclassified)
- Roger W. Stump, The geography of religion: faith, place, and space, Rowman & Littlefield Publishers, 2008, ISBN 9780742510807, 381f.
- Pradip Ninan Thomas, Strong Religion, Zealous Media: Christian Fundamentalism and Communication in India SAGE Publications Ltd, 2008 ISBN 9788178298344, 142-145.
- Patrick Johnstone, John Hanna, Marti Smith, Praying Through the Window III: The Unreached Peoples, YWAM Publishing, 1996, ISBN 9780927545983.