Population Change (Urban Places)
These graphs provide underlying detail for each urban area and rural centre as defined by Statistics New Zealand. The separation of net migration and natural change provides a clear picture of how these components interact to determine total population change. The time series makes it possible to identify trends, and is particularly important for monitoring natural change. For most urban places, natural increase is tracking downwards overall; however, this is not the case for each age group. For most towns and rural centres the natural change at older ages is increasing, due to increasing longevity and population ageing—resulting in smaller earlier-born cohorts being replaced by larger later-born ones. By contrast, the net migration trends for the different age groups vary considerably by urban area. This information can be used to determine what age groups are attracted to a town or conversely what age groups are moving away. Towns can use this information to plan and strategize accordingly.
Urban Places Methodology
The data were generated using three main sets of data originally sourced from Statistics New Zealand:
- Estimated Resident Population by age, sex and meshblock for the census years 1976, 1981, 1986, 1991, 1996, 2001, 2006 and 2013;
- Age-Specific Fertility Rates by age and territorial authority area of mother, for the years 1997, 2001, and 2006-2013 inclusive; and
- Survivorship Rates by age, sex, and territorial authority area for the life table periods 2005-2007 and 2012-2014.
All data were re-based to 2013 geographic boundaries.
Population data: Mesh-block level counts of the usually resident population by sex and 5-year age group (to 80+ years) for all census years 1976-2013 were sourced from Statistics New Zealand and aggregated to the 2013 geographic area boundaries at urban area (UA) level, based on their urban area and rural centre status in 1976. The allocation to 2013 geographic areas was based on a ‘user-derived correspondence’. The counts are not official statistics but should be thought of experimental estimates intended for use in research. This exercise resulted in data for 143 urban areas and 132 rural centres.
Birth and survivorship rates for all years for which these data were required (i.e., back to 1976) are not available at urban area or rural centre level, and were instead constructed using indirect standardisation. In order to construct birth rates, a customised dataset of births by 5-year age group and territorial authority area of mother was purchased from Statistics New Zealand (2016), covering the period 1997-2013 (June years) at 2013 geographic boundaries. Survivorship (Lx) rates by age and sex for each territorial authority area were accessed for the years 2005-07 and 2012-14 (the only years for which these data are available) (Statistics New Zealand 2015a).
Calculating missing birth rates via indirect standardisation was done in two main steps. First, age-specific fertility rates were constructed for each of New Zealand’s 67 territorial authority areas (TA) for the June years 1996-97, 2001-02, and 2006-2013 inclusive, using number of births by age of mother as sourced above, and female estimated resident population counts for corresponding 5-year age groups 15-49 years sourced from Statistics New Zealand (2015b). The age-specific fertility rates for 1996 and 2001 were then summed and averaged (for each age group and each TA), and their ratio to the equivalent rates for total New Zealand constructed (drawing on Statistics New Zealand 2015c). These relative age-specific fertility ratios for each TA were then held constant and multiplied by the equivalent rates for total New Zealand for the missing years, 1976, 1981, 1986, and 1991. That is, the national values were retrospectively inflated or deflated by the relevant ratio, for each of the four observations 1976-1991, to generate approximate TA level age-specific rates for those years.
The second step involved constructing age-specific fertility rates for each town and rural centre, by applying the age-specific rates for the TA in which each is located to the number of women in each 5-year age group 15-49 years, in each town and rural centre (from the population database).
The resulting birth rates and numbers at TA level differ slightly from those published by Statistics New Zealand (2015d) because they are constructed experimentally. As with the underlying population counts, the fertility rates and resulting birth numbers should be thought of as best approximations extracted for these research purposes.
Calculating missing survivorship rates via indirect standardisation was similarly done in two steps. First, Lx values (the average number alive in each age group) by 5-year age group and sex for each TA for two Life-Table periods, 2005-07 and 2012-14, were compared to the average number alive in the preceding 5-year age group. This process produced sex- and age-specific survivorship ratios for each five-year age group to 95+ years, for these two observations (for the purposes of this exercise, considered to be 2006 and 2013). The 2006 ratios were then compared with their national equivalents, to generate relative survivorship ratios for each TA for the missing years: 1976, 1981, 1986, 1991, 1996, and 2001. That is, for each of those observations, the national values were retrospectively inflated or deflated by the relevant sex- and age-specific survivorship ratios for each TA in 2005-2007, to generate approximate TA level rates.
The second step involved constructing sex- and age-specific survivorship rates for each town and rural centre, by applying the rates for the TA in which each is located, to the number of males and females in each five year age group, in each town and rural centre (Database 1). In order to survive age groups above 80 years, the 80+ year age group from Database 1 was prorated to 80-84, 85-89, 90-94 and 95+ years according to the New Zealand distribution (by sex) at those ages.
Again, the resulting data are ‘best approximations’ based on calendar year survivorship ratios and census usually resident population counts.
When the resulting data are compared with published birth and death numbers for each TA, which are available for all years 1992-2013, there is strong correspondence, and the model is thus considered sufficiently robust to use for our purposes of calculating the components of change for towns and rural centres. This is done using cohort component analysis and the ‘residual’ method for separating net migration from net change (e.g., Rowland 2003, Chapter 12).
Calculating components of change by the residual method: The resulting fertility and survivorship rates were used in a conventional cohort component analysis to separate out the contributing effects of both net migration and natural increase. First, survivorship rates for each age group were applied to the baseline usually resident population numbers for each individual observation (separately by sex), and fertility rates applied to survived women aged 15-49 years. The resulting births were summed and apportioned male/female according to the standard sex ratio for New Zealand (105 males per 100 females). Births were entered at age 0-4 years, and all other age groups ‘aged’ by five years. The resulting ‘expected’ population by age and sex was then compared to the actual population at the relevant observation (for example, the survived and ‘reproduced’ population from 1976 was compared to the actual population for 1981), and the difference at each age (five-year age group) taken to be a residual measure of net migration by age across the five-year period. Subtracting total estimated migration from net change in population size between the two observations in turn generates the natural increase component, which in turn is disaggregated into its births and deaths components by summing each individual component generated at each step.
- [These data originated from the Royal Society Marsden project: Tai Timu Tangata: Taihoa e? (English trans. The ebbing of the human tide. What will it mean for the people?)(The subnational mechanisms of the ending of population growth: Towards a theory of depopulation) [Contract MAU1308]. For key output from this project using this data set, see: http://igps.victoria.ac.nz/publications/PQ/2017/PQ-Vol-13-Supplementary-2017.pdf.]
- [Disclaimer: Access to these data was provided by Statistics New Zealand under conditions designed to give effect to the security and confidentiality provisions of the Statistics Act 1975. The results presented in these tables are the work of the author/s, not Statistics New Zealand.]
- [Lx values are a statistical function of the Life Table, via which life expectancy is calculated.]
Brabyn L (2017) Declining Towns and Rapidly Growing Cities in New Zealand- developing an empirically-based model that can inform policy. Policy Quarterly. 13, 37-46. http://igps.victoria.ac.nz/publications/PQ/2017/PQ-Vol-13-Supplementary-2017.pdf.
Jackson NO, L Brabyn, D Maré, MP Cameron and I Pool (In Press) From ageing-driven growth towards the ending of growth. Subnational population trends in New Zealand, in J Anson, W Bartl, A Kulczycki (Hg.) International Approaches to the Study of Population Dordrecht: Springer.
Jackson NO and L Brabyn (2017) The mechanisms of subnational population growth and decline in New Zealand, 1976-2013’ Policy Quarterly Supplement 13: 22-36. http://igps.victoria.ac.nz/publications/PQ/2017/PQ-Vol-13-Supplementary-2017.pdf.
Jackson, NO, L Brabyn and D Maré (2016) New Zealand’s towns and rural centres 1976-2013 – experimental components of growth’. Working Paper No. 7. National Institute of Demographics and Economic Analysis, University of Waikato, Hamilton New Zealand. Now being revised for AJRS.
Rowland DT (2003) Demographic Methods and Concepts Oxford: Oxford University Press.
Statistics New Zealand (2016) Customised database. Estimated birth occurrence, at 30 June 1997-2013 by Age of Mother and Territorial Authority Area of Usual Residence based on 2013 boundaries.
Statistics New Zealand (2015a) Life Table Functions 2005-07 and 2012-14, by Territorial Authority Area, age and sex.
Statistics New Zealand (2015b) Subnational population by age and sex, June 1996, 2001, 2006, 2013.
Statistics New Zealand (2015c) Live Births per 1,000 Women by Age, Maori and Total, Table DFM019AA.
Statistics New Zealand (2015d) Number of Births, Maori and Total, by Territorial Authority Area, Table VSB011AA.