Characteristics of Demographic Simulation
CONTENTS
- Basics:
- The logic of
the simulation
- Data available
for the simulation
- Discussion of data files and run parameters
- Details of packaged sets of data files
- View of data files
- Glossary
- Bibliography
BASICS: Population pyramids, rates of birth, death and migration
- Population pyramids show the size, age and sex distribution of populations
Births - infants are added at the bottom of the pyramid
Deaths - people are removed at all levels of the pyramid, especially at the bottom and top
Migration - people are removed at all levels of some populations (especially young adults), and added into other populations
Expectation of life at birth (same as average age at death) - reflected in the proportion of old to young persons.
THE LOGIC OF THE SIMULATION:
1. A MODEL OF FORCED MIGRATION.
The figure below shows a simplified version of the demographic logic of enslavement and slave trade. At the beginning of each period, we assume the existence of discrete groups known as Source and Captor populations. The Captors enslave some people from the Source population each year. In addition, we assume there are groups who were enslaved earlier: the Domestic slaves (in Africa) and the Export slaves (in the Americas).-
The people taken from
the Source population each year are known as Captives. The term "captives
refers to persons in the high-mortality period of the first year following
their enslavement, after which they become "slaves." Captives
undergo an initial mortality, and are then divided into those retained
for local use as Domestic slaves, and those intended for export from
the region. The latter group undergoes an additional mortality (corresponding
to the Middle Passage).
In the meantime, all
of these populations experience normal births and deaths. At the end
of each cycle of enslavement, the surviving Captives are added into
the Domestic and Export slave populations, and the next cycle of enslavement
begins.
The Simulation Model in Schematic Form
2. THE SIMULATION. The simulation consists of a computerized implementation of the qualitative model summarized above. This simulation of the demography of African migration through slave trade and slavery enables the user to project the effects of various conditions on the size and composition of the populations involved. The procedure relies on standard demographic techniques: it approximates a continuous analysis of population change by dividing the populations into five-year age groups and assuming that all events take place at the midpoint of five-year periods.
-
As initial
conditions, the simulation assume the discrete groups known as Source
and Captor populations. Members of the Source population are enslaved:
a given percentage of each age and sex group becomes captive each year.
Captives undergo an intial mortality, and are then divided into those
retained for local use as slaves, and those intended for export from
the region, who then experience an additional mortality as they are
sent overseas. For each of these steps, the simulation accounts for
the age and sex distribution of events. The simulation keeps track of
Source and Captor populations, Domestic and Export slave populations,
the number of captives each year, and the number of births and deaths
for each group.
The user
may select varying input data files and three run parameters, as follows:
- Source and Captor populations: fertility and mortality rates, and intrinsic net growth rates
- Captive population: size, age, and sex composition, and mortality rates
- Domestic and Export slave population: fertility and mortality rates, and intrinsic net growth rates
- Length of simulation (default is 40 years)
- Frequency of enslavement: annually, five years on and five years off, etc.
-
You may
run the simulation with the default data files. These data have been
selected to estimate the average overall impact of the Atlantic slave
trade in the late eighteenth century. The user may select alternative
input data in one of two days. New users are encouraged to select data
by packaged sets of data files, each of which is labelled by
its main difference from the default or average data. More experienced
users may with to select data by individual data files, enabling
you to select precisely the input data you wish.
3. MAKING HISTORICAL PROJECTIONS. Relating the simulation to history requires some additional steps. The populations in the simulation are of arbitrary size (100 for Source and 33 for Captor populations, initially). The work of scaling these simulated populations to fit with estimates of historical African populations involves several further steps (see Bibliography). But you may use your intuition and some rough estimates for a start. The number of Africans crossing the Atlantic each year was 60-80,000 during the late eighteenth century, and the population of the region of West and Central Africa from which they came was probably between 20 and 25 million.
