Population Projections

The standard used in projecting populations is the cohort-component method. This method projects a population in a way that duplicates the manner by which populations actually grow or decline. Much simpler methods, such as applying an assumed growth rate to a total population figure are much less satisfactory. Each “cohort,” or age group, is carried forward in time subject to the mortality experience to which it has been exposed. Thus, the age group 10-14 is carried forward for a period of time subject to relatively low level of mortality while the 70-74 age group is naturally subject to much higher mortality rates. These calculations are performed by sex since sex-specific mortality can have significant variation. Mortality is one component of change. The second component is fertility, or the number of births that women of childbearing age will have at an assumed birth rate. Projected births fill in the youngest age group in each time period and are, in turn, subject to rates of infant and child mortality. The third and final component is migration by age and sex in each time period. Migration may be positive, net immigration, or negative, net emigration. Migration is nearly always the most difficult component to estimate and to project.