What are the limitations of the data?
The National Prisoner Health Data Collection (NPHDC) was designed as a census, capturing data on the population of interest at a given point in time. However, numbers in this report represent a sample in this data collection, and not the entire prison population. Not all people in prison (particularly prison entrants and dischargees) could be asked to be involved in the data collection. This might be due to prison staffing constraints, physical or mental limitations of people, or uncertain release dates. Of those who could be approached, some did not provide consent to participate. As a result, the NPHDC sample is not necessarily representative of the total prison population. No significance testing was undertaken during data analysis.
The majority of the data collected for the Prison entrants and Prison dischargees forms were self‑reported, that is, participants (prison entrants or prison dischargees) answered the survey questions administered by health professionals or researchers. This is a simple and efficient method of collecting data, which provides the direct perspective of the person being interviewed.
Logistical advantages of this method are that interviewers do not need specialised training, and that it is generally quicker than diagnostic interviewing (for health conditions).
The main disadvantage of self-reported data is that there are few ways to validate the responses beyond excluding those where the answers given were impossible, such as where a form might have the participant’s age recorded as 1,000. Self-reported data may be compared with other self-reported data (provided where possible throughout this report) but are not directly comparable with data in reports and studies that use other data collection methods.
Comparisons are made between the health of people in prison and those in the general community where data were available. However, the data are not directly comparable due to different survey sampling techniques (random versus non-random samples) and substantial differences in the demographic profile of people in prison and of those in the general community. Comparisons are included to provide additional context and are made by sex, Indigenous identity, or age group (where possible) to help reduce the likelihood that differences were due to demographic factors rather than being true differences between people in prison and in the general population. No significance testing has been conducted on any comparisons presented.
This report does not make comparisons with data in previous reports on The health of Australia’s prisoners due to changes to questions and/or response categories over time. Data in the NPHDC are not from a random sample so the coherence of the data across survey cycles is difficult to assess and biases may be present due to sampling error.
A future aim of the NPHDC is to collect much of the data through by-products of jurisdictional administrative systems, rather than as the current entirely separate data collection. This would allow for a much larger and more representative sample, expanding the options for data analysis, and improving the validity and reliability of the collection. But there is substantial complexity involved in adapting the data requirements of the NPHDC to the different administrative data systems in each jurisdiction.
Further information on data collection methods and limitations is available in the Data quality statement.