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2.2 Populations of interest

Two study populations were used in the analysis. A sample birth cohort was defined based on people who were born between 1 July 1990 and 30 June 1991 and who were therefore aged 22 as at 30 June 2013. This single 'cohort population' was tracked over time and their risk of poor future outcomes estimated at each year of age from 15 to 22, using the regression modelling approach described below. Characteristics at ages 23 and 24 were not able to be observed for the 1990/91 birth cohort. Information observed at age 22 was used at these ages. Future outcomes were projected beyond age 24 using statistical matching to individuals from an earlier birth cohort, as discussed below.

A second study population was also used, capturing people who were aged 15 to 24 on 31 December 2013, were eligible to live in New Zealand on a permanent basis and were living in New Zealand for at least six months during 2013.[8] Once the modelling, clustering and target population definition processes were undertaken on the cohort population, the December 2013 (or 'current') population was used to describe the characteristics of the identified target populations. This current population provides a better view of the size and characteristics of at-risk individuals at a recent point in time than the 1990/91 birth cohort population would.

The birth cohort population was based on people born in 1990/91 because the coverage of the various data sets included in the IDI meant their characteristics and outcomes could be tracked up to the age of 22, covering most of the ages of interest in the Youth Funding Review. This was also the first birth cohort for which near-complete school enrolment data was available from the Ministry of Education covering the years when the children were aged 14/15 and above (ie, 2006 and subsequent years). The selection of a cohort based on a 1 July to 30 June year is also consistent with the practice of aligning age to school years.

The criteria for the 1990/91 birth cohort population were intended to select all children who were living in New Zealand as permanent residents during the 2003 to 2007 period, when they were aged 12/13 to 16/17. We selected children who met at least one of the criteria of:

  • being enrolled at a New Zealand school as a domestic student for some or all of the years from 2003 to 2007
  • having an income tax payment record in 2005-08
  • having a benefit paid to them or on their behalf in 2005-07
  • being part of the National Health Index population in 2006-07.

In addition, they had to:

  • be in New Zealand for at least three years of the period from 1 January 2003 to 31 December 2007 (in total, rather than continuously)
  • be born in New Zealand or have permanent residence entitlement through some other means (those with temporary residence visas were excluded).

Defining the birth cohort population in this way has these effects:

  • We miss a small number of children purely because a link could not be established between their administrative data records.
  • We do not include people who were away from New Zealand for much of 2003 and 2007 but were continuously resident at earlier or later phases of their lives.
  • We include some people who were overseas for a substantial part of their childhood or young adulthood. These individuals will be missing from the administrative data sets in earlier and/or subsequent years and will appear to have had no contact with the welfare, child protection or corrections systems. We are able to identify when these people were overseas but do not remove them from the study population.

The second study population comprises children and youth who were aged from 15 to 24 years at 31 December 2013, who had New Zealand citizenship or permanent residence entitlements and were living in New Zealand for at least six months during 2013.[9]

There are 289,540 people aged 15 to 19 and 292,210 people aged 20 to 24 in our current population. These numbers represent 93% and 91% respectively of Statistics New Zealand's estimates of the resident populations in these age groups in the December 2013 quarter. Our study populations are smaller because we exclude temporary residents, those who were out of New Zealand for six months or longer in 2013 and those who could not be linked to the key data sets in the IDI.


  • [8]Young people are also excluded if they had no records in the Ministry of Education data or are aged 19 or older and had no records in the Inland Revenue data.
  • [9]Young people were also excluded if they had no records in the Ministry of Education data or were aged 19 or older and had no records in the Inland Revenue data.
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