Gabriele Sommer, 'A Survey on Language Death in Africa', in Brenzinger (2012) Language Death, p. 351. See Hammarström, Harald; Forkel, Robert; Haspelmath, Martin, eds. (2017). "Hamba". Glottolog 3.0. Jena, Germany: Max Planck Institute for the Science of Human History.)
Hammarström, Harald; Forkel, Robert; Haspelmath, Martin, eds. (2017). "Okwa". Glottolog 3.0. Jena, Germany: Max Planck Institute for the Science of Human History.