The opposite applies to foreign armed forces in the United Kingdom. The projections are made for successive years running from one mid-year to the next. For each age, the starting population plus net inward migrants less the number of deaths produces the number in the population, one year older, at the end of the year. To this has to be added survivors of those born during the year. Age is defined as completed years at the last birthday.
Migration is assumed to occur evenly throughout the year.
For computing purposes, this is equivalent to assuming that half the migrants in a given year at a given age migrate at the beginning of the year and half at the end of the year. The number of deaths in a year is obtained by adding half of the net inward migrants at each age to the number in the population at the beginning of the year and applying the mortality rate q x . The total number of births in a year is assumed to be divided between the sexes in the ratio of 105 males to 100 females, in line with recent experience. The numbers of births for the projections are obtained by applying the appropriate fertility rate to the average number of women at each age during each year of the projection period. Because cohort fertility rates are more stable than those for calendar years, the fertility rates used in the projections are derived from assumptions relating to the year in which women were born.