2.1.4 Population projections
The series for fertility, mortality, and migration were combined to give four population variants, as shown in Table 2. The projections were carried out using the standard cohort component projection methodology (Preston et al 2001). Data on the age-sex structure in the base year were obtained from the 2001 Census.[3]
| Variant | Fertility | Mortality | Migration |
|---|---|---|---|
| Base | Base | Base | Base |
| Low fertility | Alternative | Base | Base |
| Low mortality | Base | Alternative | Base |
| Low migration | Base | Base | Alternative |
Figure 4 shows trends in total population for the four variants. As the low-fertility variant illustrates, a sharp decline in fertility would entail rapid population decline. The low mortality variant portrays an interesting situation: constantly-improving mortality and a small migration surplus more than compensate for a fertility rate slightly below replacement level, so that population growth continues to be positive.
Figure 5 indicates the changes in age structure associated with each of the four variants. The population share of 0-19 year olds is virtually the same in all variants except the low-fertility variant, where it falls quickly and remains at low levels. Both the low-fertility and the low-mortality variants are associated with large increases in the population share of older people. (The idea that low fertility would raise the proportion in older age groups can appear strange; the explanation is that a fall in birth rates raises the ratio of those born many years previously to those born recently.) The population share of young people and old people is very similar under the base and low-migration variants. This is partly because the alternative migration assumption is relatively moderate, but also because small changes to migration rates do tend to affect population size more than they affect population structure.
In the base, low fertility, and low migration variants, the proportions in each age group stop changing by the end of the projection period. These populations have reached the condition known in demography as ‘stability’ whereby, in response to a sustained period of constant fertility, mortality, and migration rates, all age groups are growing or shrinking at the same speed. The low-mortality variant does not reach stability since mortality in this case is not constant.
- Figure 5 - The percentage shares of selected age groups in the total population
- Percent of population aged 0-19
- Percent of population aged 65+
Notes
- [3]Data obtained from the Statistics New Zealand website. Statistics New Zealand’s tabulated results combined aged 85 and above. These were disaggregated using the extended life tables described in Appendix 1 and an assumption that the population was stable.