Long-Term Insights Briefing for the justice sector

Methodology

Descriptive analysis
The Long-Term Insights Briefing relies on descriptive analysis. Time series, percentage changes and rates of various descriptions are used to describe imprisonment in New Zealand. No statistical techniques were used to establish correlations or to make causal inferences.

The descriptive analysis is as good as the data it relies on and the capacity of the analysts to organise the data and draw out statistics that accurately represent the imprisonment regime in New Zealand over 60 years. For the most part the data is considered to have high integrity, the most significant caveat is in relation to ethnicity data. There is no way of knowing how well ethnicity data was collected in the past, and there continues to be inconsistent recording and reporting of ethnicity across the justice sector today. There is also no consistent long-term count of Māori in the general population.

The prime focus is on the prison population: the number of people in prison at any point in time. Other measures, such as the rate of imposition of prison sentences, the length of those sentences, rate of remand in custody and time spent on remand are mainly used as an aid to understand changes in the prison population.

One of the most important units of measurement used is the imprisonment rate, this term can mean different things to different people and can be used differently in different contexts. It can be used to describe the proportion of people appearing for sentence who are imprisoned, that is not the usage here, we have termed this “the rate of use of imprisonment”. The rate of imprisonment employed here is the number of people in prison per 100,000 people in the population of interest. This measure is commonly used around the world. It enables comparisons to be more easily made between groups.

Often imprisonment rates are calculated against the general population. This is helpful at a high level but can be misleading if the populations being compared have different structures. The most significant structural difference is age. This is significant for the current analysis because the likelihood of being in prison is typically greater for younger adults. Consequently, if the age structure of the general population is older or younger than a comparison population, variations in imprisonment rates may be more reflective of different age structures than actual imprisonment rates.

It is almost always the case that some data is missing or unknown, this is particularly the case with demographic data. In those cases where data was missing or there were positive counts in categories like “Unknown” or “Not Stated” the observations were omitted from the analysis.

The data
Data on the prison population The data on the prison population was drawn from three sources, each covering a distinct period.

For the period from 1960 to 1979 data was drawn from the annual Justice Statistics reports published by the Department of Statistics. The available data was quite rudimentary, providing the total annual prison population with separate counts for the sentenced prisoner and remand prisoner populations. The data could be used to identify eight demographic subpopulations based on gender (men/women) and ethnicity (Māori/nonMāori). The accuracy of the attribution of ethnicity is unknown. There was no age, offence, or sentence information available.

For the period from 1980 to 1999 data was drawn from ‘major management periods’ data held by the Department of Corrections.

This data came from the case management sub-system of the Law Enforcement System (known to many as the ‘Wanganui Computer’). The data was richer, it included a wider view of ethnicity (Māori, European, Pacific and other), age (six age bands were specified), and offence information (a three-level structure commencing at a high level of
abstraction was specified). The data only covered the sentenced prisoner population, the source data did not cover remand prisoners. Data on the number of people on remand was able to be found in other publications (Justice Statistics and its successor the annual Conviction & Sentencing series originally published by the Department of Justice and continued by the Ministry of Justice) but there was little detail available.

For the period from 2000 to 2022 data was drawn from the Department of Corrections Enterprise Data Warehouse which holds demographic information, current and past sentences and offence information relevant to the Long-Term Insights Briefing. The data was able to be organised at unit level (per person at 30 June each year).

The ability to draw time series and describe and analyse the prison population was constrained by the scope of the data. A 62-year series could be drawn but the detail was limited. A 42-year series could be drawn for the sentenced population with considerable ability to specify more than 1,000 sub-series based on age, gender, ethnicity and offence type. A 22-year series could be drawn with additional sub-series bringing in the remand population and some sentencing details.

The population counts did not match perfectly at the transition points (1979/1980 and 1999/2000) but the differences were not large and the general trends were reasonably consistent.

Data on other criminal justice measures

The Ministry of Justice provided datasets on prosecutions, convictions and sentences (type and imprisonment sentence length) for the years to 30 June from 1981 to 2021. This data also had its origins in the case management sub-system of the Law Enforcement System and then from the Ministry’s Case Management System.

The data is based on what is described as a person-day case. That is it brings together all charges laid against a person on the same day into a single prosecution and all charges against a person that were sentenced on the same day into a single case. The prosecution and sentenced cases do not match. This method results in an overcount of prosecutions and sentencing but it is a consistent overcount and, therefore, trends and time series are reasonable.

Data on sentence lengths, time served on remand, time served as a sentenced prisoner, proportion of sentenced served were sourced from the Department of Corrections Enterprise Data Warehouse.

Information on Police proceedings used in the section on young adults came from the Police website (https://www.police.govt.nz/about-us/publications-statistics/data-and-statistics/policedatanz/proceedings-offender-demographics).

Data on the general population

The data on the general population used to create rates of imprisonment was drawn from Statistics NZ’s website. Data on the total and Māori populations were sourced from the Infoshare tool making use of the ‘Population estimates’ located within the Population. The non-Maori population estimate had to be derived by subtracting the estimate for Māori from the general estimate.

The estimates run from 1991. Consideration was given to using other sources for earlier years but given a substantial change to the way the Māori population was counted in the census prior to 1986 this approach was rejected. Prior to 1986 people were only counted as Māori if they had at least 50% Māori blood. In the 1986 census the method changed with a person counted as Māori if they selfidentified as Māori.

There is no equivalent source of information on Pacific Peoples. There is an annual series that has a count of Pacific People that includes everyone who identifies as having any Pacific heritage. Unfortunately, the prison population does not have an equivalent count. Within the NZStat tool under the ‘Population estimates’ tab within the ‘Population’ tab there are estimates based on ethnicity for 1996, 2002, 2006, 2013 and 2018. This data was used to calculate imprisonment rates for Pacific Peoples.