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Data sources

The New Zealand Census of Population and Dwellings

The New Zealand Census of Population and Dwellings is the official count of people and dwellings in New Zealand. It provides a snapshot of New Zealand at a point in time, and measures social and economic change. The latest census was held in March 2013. 

The census aims to count everyone who is in New Zealand on census night. Overseas visitors are included in the census, while New Zealand residents who are not in New Zealand on census night are not included.

Not all of those counted in the census returned census forms. The census count includes 4.8 percent (203,052) substitute records (Statistics NZ, 2014b). A substitute is a census record that is created where there is sufficient evidence received during the collection process that a person exists or a dwelling was occupied, but we obtained no corresponding form. As such, they form part of census non-response.

Some people are missed altogether or counted more than once in the census. Coverage in the census is measured by the Post-Enumeration Survey (Statistics NZ, 2014b). Net census undercount for the 2013 Census was estimated at 2.4 percent. Younger adults aged 15–29 years had a higher percentage undercount (4.8 percent) than other age groups.

The estimated resident population (ERP)

The ‘estimated resident population’ of New Zealand is an estimate of all people who usually live in New Zealand at a given date (Statistics NZ Standard for population terms).

The estimated resident population of New Zealand is derived by adjusting the census usually resident population count for net census undercount (as estimated by the PES) and the estimated number of residents temporarily overseas on census night. To obtain the estimated resident population at a given date after census night, updates are made for natural increase (births less deaths) and net migration (arrivals less departures) between census night and the given date. The official estimated resident population (ERP) series provides the best measure of who is living in New Zealand at a given date.

The ERP is at its most accurate immediately after the most recent census, and accuracy generally decreases over time the further we move away from the census. For this reason we use the official ERP in the base census year at 30 June 2013 as the comparison for population estimates constructed from the linked administrative data in the Integrated Data Infrastructure (IDI).

The Integrated Data Infrastructure (IDI)

Statistics NZ has developed the IDI as an environment in which to link multiple data sources in a systematic and secure way. It was developed to produce official statistics outputs and to allow Statistics NZ staff and external researchers to conduct policy evaluation and research on people’s transitions and outcomes. The IDI contains administrative and survey datasets, linked at the individual level. We use the IDI as a test environment for examining the potential of linked administrative data sources to produce population estimates.

The IDI continues to change as new datasets are added (see current information at stats.govt.nz/idi). This section describes the structure and content of the IDI as at May 2015.

The basic structure of the IDI is shown in figure 1. The structure of the IDI can be described as a central 'spine' to which a series of data collections are linked.

The spine forms the conceptual centre of the IDI. Broadly, the target population for the spine is all individuals who have ever been residents of New Zealand. The spine aims to include each individual only once.

Three data sources are linked together probabilistically to create the spine:

  • a list of all IRD numbers that have been issued by Inland Revenue
  • a list of all births registered in New Zealand since 1920
  • a list of all visas granted to migrants from 1997 (excluding visitor and transit visas).

The spine is the mathematical union of the three contributing data sources. People present in at least one source will be included in the spine. The linkages between the three contributing data sources ensure that people present in any two data sources are included only once in the spine.

Other data sources are linked to the IDI spine (see Statistics NZ 2014c for a description of the linking process). The linked datasets cover a wide range of subject areas and include: employer and employee job and earnings information based on Inland Revenue data; health information including GP enrolment and hospital visits from the Ministry of Health; education data from the Ministry of Education; benefit dynamics data from the Ministry of Social Development; student loans and allowances data from several sources; migration movements data from the Ministry of Business, Innovation and Employment; and the Household Labour Force Survey and New Zealand Income Survey from Statistics NZ.

The IDI also contains several summary tables that provide core information about individuals (age, sex, ethnicity, and geographic information) summarised from across the available data sources.

Using the IDI to determine who is living in New Zealand

The IDI spine forms our starting point for determining who is living in New Zealand. Many of the other data sources contain information about events or activities where people interact with government, and can indicate who is present in the country at a given time.

External migration is very important in this context. The MBIE 'border movements' data in the IDI records travel journeys into and out of New Zealand. When linked to individuals in the spine, the border movements can help to indicate when people have left New Zealand on a long-term basis, and should no longer be counted among the New Zealand residents.

Deaths data is also linked to the spine, and allows us to remove those who have died from the population.

Figure 1 
Structure of the Integrated Data Infrastructure in May 2015

Diagram showing structure of the Integrated Data Infrastructure in May 2015.

The linked Census-IDI

The 2013 Census has been linked to the IDI. The linked Census-IDI dataset used for this study was created by the Census Transformation programme, and was created to better understand the coverage and quality of census information in the IDI. The linked data was only available to approved Statistics NZ staff working on Census Transformation.

The census was linked to the spine of the IDI in the May 2015 IDI refresh. Linking was completed in Quality Stage using probabilistic matching techniques. The variables full name, date of birth, sex, meshblock of usual residence, and country of birth were used in the linkage process.

Overall, 92.4 percent of the census usual resident population count were linked to the IDI. Of most interest here, 95.4 percent of people from responding households (ie excluding those where substitute records were used for the entire household) were linked to the IDI. The linkage rate was better for individuals who had used electronic forms (98 percent linked) compared with paper forms (93 percent linked).

In any probabilistic linking process there will be linkage error. A false positive linking error occurs when two records are linked but they are not a true match (the linked records belong to different people). There is an estimated false positive rate of less than 1 percent in the Census-IDI dataset.

A false negative linking error occurs when two records are a true match (they belong to the same person) but they are not linked. False negative errors are more difficult to detect. The linkage rate of 95.4 percent of census respondents provides an upper bound of 4.6 percent for the false negative linkage rate.

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