Cohort-component population projections (cont.)

Calculating  the births (cont.)

To partition these births into boys and girls, requires an estimate of, or an assumption about, the sex ratio at birth. This is usually about 105 boys for every 100 girls. Using this estimate, one can calculate the number of female and male births respectively, as:

In Sri Lanka this gives an estimate of the girls born between 2010 and 2015 of:

Note that this method for projecting births produces what is termed a female dominant projection. This is because the initial size of each age cohort of both girls and boys is calculated by applying age-specific fertility rates for women to the projected population of women.

In principle, no reason exists not to carry out a male dominant projection except that reliable age-specific data on men’s fertility are very rare. Either approach ensures that the numbers of boys and girls that are born remain at the assumed ratio (or series of ratios) throughout the projection and, therefore, that the male and female populations grow at the same rate in the long run. If one was to carry out completely independent projections of the male and female population, nothing in the computational procedure would prevent their sizes from diverging in ways that are biologically implausible.