Cohort-component population projections (cont.)

Adjusting for migration

Several different approaches can be used to incorporate migration flows into population projections. The most appropriate approach to use depends in part on the data on migration that are available. Because immigration and emigration are difficult to measure and often fluctuate sharply and erratically, simple approaches may perform just as well as more sophisticated methods.

In principle, if one can forecast age-specific emigration rates, then emigration can be dealt with in exactly the same way as mortality by applying life table probabilities of not emigrating to each age cohort.

Immigration is different as there is no population at risk of immigrating (other than the global one). Therefore, no good reason exists to do anything more complicated than add an estimate of the number of immigrants in each age group to the projected population at the end of the interval.

Many projections go further than this and simply add estimates of net migrants to the projected population rather than trying to model the larger gross flows of emigrants and immigrants. This approach is adopted here. Other ways of incorporating migration into projections are discussed in module PAPP103.

Thus, using this approach, the final projected number of women aged 5-9 years in Sri Lanka is calculated by subtracting net emigrants from the survivors at the end of the interval (calculated as 924.6 on the previous page):

P(5,2015) = P (0,2010) × S(5,2010) + NM(5, 2010) = 924.6 − 2.6 = 922.0

Interaction This number appears in column 5 of the results. Place your cursor over the formula to see this.

Age group Female population in 2010 ('000s) Life table person-years – 5Lx (e0=77) Net migrants ('000s) Female population in 2015 ('000s) Age-specific fertility rates Births by age of mother
 0–4 929 486743 −2.4 873.4    
 5–9 856 484440 −2.6 922.0    
10–14 764 483819 −2.3 852.6    
15–19 797 483213 −2.5 760.5 0.0222 86.4
20–24 818 482354 −4.1 791.5 0.0938 377.4
25–29 873 481210 −5.1 811.0 0.1502 632.3
30–34 821 479708 −3.5 866.8 0.1155 487.3
35–39 733 477656 -2.1 815.4 0.0533 206.3
40–44 732 474667 −1.3 727.1 0.0138 50.3
45–49 689 470169 −0.8 724.3 0.0012 4.2
50–54 641 463275 −0.8 678.1    
55–59 527 452580 −0.8 625.4    
60–64 454 435860 −0.7 506.8    
65–69 338 409120 −0.5 425.6    
70–74 252 367295 −0.3 303.1    
75–79 170 307729 −0.2 210.9    
80–84 102 228984 0.0 126.5    
85+ 66 231178 0.0 84.4    
Total 10562   −30.0 11105.4 2.25 1844.2
Female births           899.6