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

Age and Gender Ratios

As noted above in description of data variables for “Age categories,” they do provide enough information to describe in gross terms the demographic structure of the trade—the age and gender composition of the Africans carried off as slaves. For 93 voyages, age and sex data is available for up to three places of embarkation and two places of landing of slaves, and for 437 voyages information exists on the age and gender of slaves who died on the crossing. When available, the quality of the data varies considerably. The most precise data is for 3,404 voyages with records that report the number of “men,” “women,” boys,” and ”girls,” as well as in some cases “children” and “infants.” For another 811 voyages, the number of “adults” and “children” are distinguished without indication of their gender; while 484 voyages reported the number of “males” and “females” they carried without specifying how many were adults and children.

Several age and gender variables were imputed from the data variables. First, information from voyages with more than one place of embarkation and/or disembarkation of slaves was aggregated to produce one set of four variables – adults, children, males, and females – for places of departure and another for places of arrival. An additional set of the same variables was calculated for deaths on the Middle Passage. From these variables, we calculated two ratios: the proportion of children for all voyages with information on adults and children, and the proportion of males for all voyages with information on males and females. The proportion of adults and females are the reciprocals of the variables that were computed. The ratios are to be understood as the proportion of all slaves for whom a characteristic can be determined. The child ratio is the percentage of slaves identified as either children or adults, and the male ratio is the percentage of slaves identified as males or females. For the smaller number of voyages with information on both age and gender, we also calculated the ratios of men, women, boys, and girls. Ratios were imputed only for voyages with at least 20 slaves whose age and/or gender is documented.

Each ratio was calculated for places of departure, arrival, and deaths; but to further simplify the information for inclusion in the set of variables available on the Voyages website, one set of ratios is provided for each voyage with age and sex information. It is the same as the proportion at arrival when that is documented; otherwise it is the proportion at departure. The age and sex of captives was recorded almost two times more often at places of landing (3,731 voyages) than it was at places of embarkation (1,970 voyages). Information on the demographic composition of slave cargoes at both embarkation and disembarkation exists for only 609 voyages.

Caution is required in inferring age and gender patterns from the ratios. Ships left the African coast with varying numbers of men, women and children on board. It makes little sense to combine, say, the Merced, taken into Sierra Leone with only one man slave on board, and the Alerta, which landed 69 men among 606 slaves disembarked in Havana in September 1818. The ratio of men in the first voyage was 100 percent, the ratio in the second case was 11 percent. Averaging without any further adjustment produces a ratio of men of 56 percent, which, given the different numbers of people on board, misrepresents historical reality. With large enough numbers of cases, this problem diminishes to the point of becoming negligible; but if users select a small number of cases, they should employ a simple weighting technique to correct for the differences in the number of people being counted. Thus, in the above example, the weighted average of men on the two ships is very much closer to the 11 percent on the Alerta than the 100 percent on the Merced. Alternatively, users might disregard our voyage-based age and gender ratios and simply divide the total of males (or females) by the total number of slaves in the sample they select.

As the above discussion suggests, the ratios for age and sex made available in the Voyages Database are calculated without weighting. For example, “Percentage male*” (MALRAT7) is computed by averaging the ratios computed for each voyage. Thus a mean of, say, 70 percent male for a group of years or a region is the unweighted average of male ratios for individual voyages in the selected group. If users wish to group all males in the selection (or all children) and divide by all slaves, they may obtain somewhat different results from those provided in the search interface, but they will have to first download the database to make that calculation.

Users should also remember that age and sex information was recorded on some vessels at the beginning of the voyage and on others at the end of the voyage. We have created composite male and child ratio variables, “Percentage male*” (MALRAT7) and “Percentage children*” (CHILRAT7) that lump together information from both ends depending on availability of data, and where information has survived on both we gave precedence to the ratios at the point of disembarkation. This procedure is justified by the finding that shipboard mortality was only modestly age and sex specific. Those users who wish to eliminate these modest effects should download that database first.(12)