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2.2  Study population

Because the IDI source data is collected primarily for administrative, rather than statistical purposes, populations can be defined in many different ways. This choice is often dictated by the research needs, building on previous approaches used. These approaches are designed to ensure that as much as possible the study population includes those people who are relevant to the research questions being addressed, while excluding those who are not of interest to the study.

This study adopts a slightly different approach to that used in the SII tool and associated papers (McLeod et al. 2015 and Ball et al. 2016). The SII tool used a range of criteria to identify a study population at the end of December 2013. For example, to be included, a child or young person had to:

  • be eligible to live in NZ on a permanent basis
  • be living in New Zealand for at least six months during 2013
  • have records in the education data (where they are young enough to do so)
  • have records in the tax data (where they are aged over 19).

Insights uses an alternative approach, based on the estimated IDI New Zealand resident population. This approach is outlined in Gibb et al. (2016), although we have modified it in a number of ways:

  • We use reference years ending in December rather than June. In addition time in the country is calculated using the calendar year, with people included in the ERP if they spent more than 6 months of the year in New Zealand.
  • Temporary migrants, such as international students and working holidaymakers, are excluded as we have limited data about their past, they may not stay in NZ long-term, and they are often not eligible for services.
  • Custodial sentences are used as an additional 'activity measure' to identify people who are in the population.
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