According to the Encyclopedia of African History, "It is estimated that by the 1890s the largest slave population of the world, about 2 million people, was concentrated in the territories of the Sokoto Caliphate.
Segundo a Encyclopedia of African History, "Estímase que na década de 1890, a maior poboación escrava do mundo, arredor de 2 millóns de persoas, estaba concentrada nos territorios do Califato de Sokoto.