Construction of the Trans-Atlantic Slave Trade Database: Sources and Methods

Coverage of the Slave Trade

One immediate question is how complete are the data? It is probable that our data set now includes more than 95 percent of all voyages that left British ports—and the British were the second largest of the national slave trader groups. The data on the eighteenth-century French and Dutch slave trades are also largely complete. The reasons for such comprehensive coverage are fairly obvious. Compared with other slave traders, northwestern European trading nations conducted the great bulk of their business relatively late in the slave trade era when everyone kept better records. Surviving sources in these countries are therefore abundant. Casual inspection of the relevant variables in the data set shows that almost all the voyages leaving ports in these countries have more than one source of information, and some have as many as eighteen. Yet the data on the Iberian and Brazilian trades after 1750 are also relatively complete, and information on the earlier period for these regions is vastly greater than it was ten years ago. For a country by country assessment of the completeness of the data, readers are referred to Chapter 1 of Extending the Frontiers (1) and the spreadsheets downloadable from this web site that underpin our estimates of the overall size of the slave trade. Our estimate of the total volume of slaves carried off from Africa to the Americas is 12.5 million, and that the total number of voyages that set out to obtain captives was 43,600. New information will certainly emerge from the archives, but we think it unlikely that future scholars will dramatically increase the size or scale of the trans-Atlantic slave trade.

The 34,948 trans-Atlantic voyages contained in the database allows us to infer the total number of voyages carrying slaves from Africa. The Estimates page suggests that 12 ½ million captives (12,520,000) departed Africa for the Americas. Dividing this total by the average number of people embarked per voyage, 304 individuals, yields 41,190 voyages. Similarly, the Estimates pages suggests that 10.7 million enslaved Africans disembarked, mainly in the Americas. Given the average number disembarked per voyage, 265 people, yields an estimated 40,380 voyages arriving. Not all 34,948 voyages in the database carried slaves from Africa. A total of 1,262 voyages (3.6%) never reached the African coast because they were lost at sea, captured or suffered some other misfortune. After removing these voyages, the database contains some trace of 81 percent of voyages that embarked captives. The database also contains records of 33,684 voyages that disembarked slaves, or could have done so (in other words, for some of these we do not know the outcome of the voyage). A total of 924 of these disembarked their slaves in the Old World. The latter group comprised mainly ships captured in the nineteenth century which were taken to Sierra Leone and St. Helena as part of the attempt to suppress the trade. A further 276 sank after leaving Africa with the loss of their slaves. In all, the database contains some record of almost 80 percent of vessels disembarking captives. Of course, there are other estimates of the volume of the trans-Atlantic slave trade. If we take a higher estimate of, say, 15.4 million departures,(2) then the Voyages Database documents two-thirds of all slaving voyages that sailed between 1514 and 1866.