Demographic projections are designed to meet both short-term and long-term planning needs, but are not designed to be exact forecasts or to project specific annual variation. These population projections are based on assumptions made about future fertility, mortality, migration, and inter-ethnic mobility patterns of the population. Although the assumptions are carefully formulated to represent future trends, they are subject to uncertainty. Therefore, the projections should be used as guidelines and an indication of the overall trend, rather than as exact forecasts.
The projections do not take into account non-demographic factors (eg war, catastrophes, major government and business decisions, changes to the ethnic classification) which may invalidate the projections. The unpredictability of migration trends, especially in the short term, can have a significant effect on projection results.
Projections of ethnic populations are more uncertain than projections of the total population for several reasons:
- Ethnic identification can change over time. This may reflect a person's cultural affiliations changing over time. Or it may occur when different people respond to the ethnicity question. For example, the ethnicity of babies and young children is usually identified by their parents. However, in a later census when these children are old enough to complete their own forms, they will decide for themselves which ethnicity they identify with. This may differ from the ethnicity identified by their parents. Inter-ethnic mobility can also occur when different ethnicities are reported in different collections (eg birth registration form, death registration form, census form) for a person.
- There are greater difficulties in establishing past trends in fertility, mortality, and migration. Different ethnicities can be reported in different collections (eg birth registration form, death registration form, census form), which makes the derivation of ethnic-specific fertility and mortality rates problematic. Furthermore, the measurement of ethnicity has changed over time in many collections, while it is not captured at all in some collections (eg international travel and migration data).
- Ethnic populations are not mutually exclusive because people can and do identify with more than one ethnicity. People are not asked to prioritise their ethnic responses. Hence, Statistics New Zealand includes people in each of their reported ethnic groups.
- There is the added complication of births to parents of different ethnicity. The child may be considered by the parents to belong to one or more of their ethnicities, or indeed to another ethnicity.
- There is greater future uncertainty about the components of population change. For example, it is uncertain whether the fertility and mortality of different ethnicities will converge, and if so, at what pace. Assumptions about future migration, notably for people of Asian and Pacific ethnicities, are particularly susceptible to changes in migration patterns.
Statistics NZ incorporates these issues into its methodology for ethnic population projections and develops alternative projection scenarios to illustrate uncertainty. However, it is because of these issues that ethnic population projections are limited to broad ethnic groups and the given projection period. For smaller ethnic populations it is difficult to derive robust measures of fertility and mortality and the other components of ethnic population change to enable projections to be readily produced. The subnational ethnic population projections are further limited to selected geographic areas because of the small size of many subnational ethnic populations.