Hogg, π. 424 κεφάλαιο 9 English Worldwide του David Crystal: "approximately one in four of the worlds population are capable of communicating to a useful level in English." Hogg, Richard (2008). A History of the English Language. Cambridge University Press. ISBN052171799X. Ανακτήθηκε στις 13 Απριλίου 2010.