In his 1969 book "The Atlantic Slave Trade: A Census", Curtin researched the sources of frequently used estimates of the number of individuals transported across the Atlantic Ocean by the slave trade. His analysis of shipping contracts and data from the ports of entry enabled him to estimate between 9 and 10 million individuals being transported on slave ships, with a margin of error of 20%, out of the 20 to 30 million that had been loaded aboard at ports in Africa. Prior to Curtin's research, estimates of the number of individuals brought from Africa as slaves ranged from 3.5 million to numbers as high as 100 million individuals. A widely cited number of 15 million slaves used by W. E. B. Du Bois, who had gotten the number from abolitionist Edward Dunbar. Another widely quoted estimate of 20 million slaves was based on calculations using data concerning slaves in Jamaica that was adjusted for the entire Atlantic slave trade, though the original data used to make the calculations has since been lost.