5.3  Migration

In principle, it would be preferable to make separate in-migration and out-migration projections, rather than a single net-migration variable. There would, however, be little material gain in exchange for the added complexity, as migration has far less influence on age-structure, and hence support ratios, than either fertility or mortality. (The reason is that migration occurs at a wide range of ages, whereas everyone is born at exactly age 0, and most people die at ages 60 and over).

This paper is somewhat unusual in that the migration projections have been carried out in terms of rates rather than numbers. For short-term projections, the choice of numbers or rates in fact makes little difference. For long term projections, it can be argued that the use of rates is more sensible. This is most obvious for out-migration, which is clearly related to the size of the population. However, for New Zealand, it is likely to be true for in-migration as well. A significant proportion of in-migration to New Zealand consists of New Zealanders returning from overseas, and the number of New Zealanders overseas is correlated with the size of the resident population. Moreover, in-migration by non-New Zealanders is at least partly determined by government immigration policies, and future governments will presumably look at the size of the population when they make immigration policy.

Appendix Figure 3 - Migration rates 1961-2001

Appendix Figure 3 shows historical net migration rates. As is apparent in the figure, net immigration has fluctuated widely, with outflows roughly balancing inflows (the mean for the period shown is about 0.5 per thousand.) In contrast to fertility and mortality rates, there is no generally accepted summary index for a schedule of age-sex-specific immigration rates. (This may be one reason why demographers have traditionally used numbers rather than rates.) In the projections we therefore resorted to a somewhat awkward index: the overall net migration rates which would result if the schedule of age-sex-specific rates were applied to the New Zealand 2001 population. In the base variant, this measure is assumed to be constant at the historical average of 0.5 per thousand from 2001 to 2171; in the alternative variant it is assumed to be 0.

Age-sex-specific net migration rates were related to the summary index using the model

(16)

where is the net migration rate for age-sex group , is the net migration rate index described above, and and are coefficients for age-sex group . These coefficients were estimated from age-sex-specific migration data for the period 1996-2001.

5.4  Consumption weights

5.5  Productivity weights