In determining "preferred ethnicity", individuals are associated only with their most recent self-identified "preferred ethnicity" as recorded in Corrections’ database and grouped according to Statistics New Zealand ethnicity groupings (see ethnicity groupings here ).
Where no ethnicity is available from Corrections' data, the data is then supplemented with historic data from the Ministry of Justice (MoJ) data warehouse. The MoJ data warehouse in turn inherited this ethnicity data from the Law Enforcement System (LES) running since mid-1970s. The LES supplementary data was apparently based on Police officers' assessment of the offender’s ethnicity. The bulk of the data (and all of it since 1998) comes from Corrections' database. Even with the inclusion of the supplementary data, there are significant numbers of individuals with unknown ethnicity prior to 1998. As far as the author is aware the missing ethnicity data is an artefact of the data collection and import processes of the time, and all ethnicities would have been equally impacted.
The "preferred ethnicity" approach used here partly follows Statistical Standard for Ethnicity 2005, and differs from the standard in the following ways:
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