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2.2  Study populations

Two study populations were used in the analysis. The first was a ‘current' population, comprising people who were aged 0 to 14 years on the 31 December 2013. This population was used to examine the relationship between the four indicators and the wider set of contemporary measures available in the IDI. The second was a birth cohort that was used to examine the relationship between characteristics observed at a given age and subsequent outcomes.

At the time the IDI based study of 15-24 year olds was undertaken most of the key datasets covered the period up to the end of 2014, however the Ministry of Education data only covered the period up to the end of 2013. The 31 December 2013 time point was chosen so that the analysis was consistent with the analysis of youth aged 15 to 24 years, undertaken in mid-2015.

The 'current' population comprises those who were aged 0 to 14 years at 31 December 2013, who were eligible to live in NZ on a permanent basis (ie, had New Zealand citizenship or permanent residence entitlement), and were living in New Zealand for at least six months during 2013.[7]

There were 362,800 people aged 0-5 and 510,400 people aged 6-14 in the current population. These numbers represent 97% and 95% respectively of Statistics NZ'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 NZ for six months or longer in 2013, and those who could not be linked to the key data sets in IDI.

For the regression analysis a birth cohort was selected which included all people who were born in 1993 who were enrolled as domestic students in New Zealand schools in 2008 or 2009 (ie, when they were aged 15 and 16 years). This cohort is observed in IDI up to their 21st birthday.

Defining the birth cohort population in this way means that a small number of children are excluded because a link could not be established between their administrative data records. We did not include people who were away from New Zealand for much of 2008 or 2009, 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 datasets 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.


  • [7]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 IRD data.
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