The statistics used in Australian Crime: Facts & Figures are derived from a variety of administrative datasets and surveys; using both types of information enables a better understanding of the incidence and effects of crime in the community. The sources used to compile this edition are listed in the References section.
Different criminal justice agencies record data at different stages of the criminal justice process. For example, police keep incident records, courts record the details of cases and their disposition, and corrections agencies have details of the offenders in their charge. Most basic information comes from these administrative collections, which have the advantages of covering the entire population that comes into contact with the criminal justice system and of remaining relatively stable in their collection and production over time.
There are limitations to these data, however, including issues of comparability among agencies and jurisdictions. In addition, it is only recently that most of the data have been collated at a national level, where collating occurs at all. Specifically, national collections of recorded crime from police records have been collated since 1996, prisoner data from corrections agencies has been collated since 1983, data for all criminal courts has been collated since 2001 and offender data has been collated by the ABS since 2009. However:
- The collections are not all based on the same unit of measurement. For example, the police record details about offences, courts record data at an individual case level and corrections agencies record information about individual offenders.
- Although much has improved, national collections are hampered by jurisdictional differences in legislation and definitions, and data collection methods that are often not uniform. Data recording quality may also be an issue.
- It can take time to reach agreement at a national level on key issues, including definitions of new and emerging offences. As a result, more detail about crime and justice is often available at a jurisdictional level, even when it is not possible to produce national statistics.
- Although most collections are grouped by year, the specific time period may vary between agencies. For instance, some follow the calendar year (January to December), while others are based on the financial year (July 1 to June 30 of the following year). Therefore, care should be taken when attempting to interpret information from incompatible time periods.
It is also worth noting that the datasets can only encapsulate reported crimes, yet not all crimes are reported to police. For example, it is estimated that only 20 percent of all sexual assaults are reported to police, compared with a reporting rate of nearly 100 percent for motor vehicle thefts (Reference 28). This is the primary reason that the other type of data collection—crime surveys—are undertaken.
Crime victimisation surveys are believed to provide a more accurate picture of actual crime rates in society as they attempt to measure all crime—including crime that has not been reported to police. They have the advantage of asking the same questions in the same way to all of a sampled population. These answers are then recorded in a uniform way so that the information they provide is both reliable and comparable.
However, it is not always valid to extrapolate from a sample to the general population and all sample surveys contain a certain amount of error. As surveys are also expensive, they generally tend to be undertaken on an infrequent or ‘one off’ basis. The surveys used in preparing this year’s edition of Australian Crime: Facts & Figures include the Drug Use Monitoring in Australia survey conducted by the AIC and Crime Victimisation survey conducted by the ABS.
Notes on using these statistics
It should be noted that police information on victims and offenders has not been tested in court. For example, an offence recorded by police as ‘murder’ may later be re-classified as ‘manslaughter’, or there may be insufficient evidence to convict an alleged offender of any criminal offence. The category of homicide includes both murder and manslaughter. Where murder is used as a category, it excludes manslaughter.
The apparently marked fluctuation in some crime rates may be due to the small number of cases involved. For example, if only four homicides have occurred in one year, the addition or removal of one homicide per year will appear as a 25 percent increase or decrease.
Rates are determined against one of two different types of base population—either the total population, or total relevant population. The property crime victimisation rate, for example, divides the number of property victims by the total population. In this publication, data in relation to the total population are presented as per 100,000. Rates in relation to a relevant population refer to the number of persons as a proportion of that population (eg juveniles, males, females, or Indigenous persons, where for the purposes of this report Indigenous refers to members of the Aboriginal and Torres Strait Islander communities).
Population projections for Indigenous adults are based on data provided by the ABS. The ABS uses two methods to estimate Indigenous populations—the low series and the high series. Both contain certain assumptions about births, deaths and migration. Figures in this publication are based on high-series population data. This method accounts for the effects of the increasing propensity for people to identify as Indigenous from the 1991 to the 2001 censuses. In 2004, the ABS released revised Indigenous population figures in the high series for 2001–03, based on the 2001 census. Rate calculations for these years therefore differ from those in some previous publications.
Data on juveniles refer to persons aged 10 to 17 years, while data on adults refers to persons aged 18 years and over. Issues of Australian Crime: Facts & Figures up to 2004 define adults as persons aged 17 years and over, affecting calculations such as rates of imprisonment. From the 2005 issue onward, imprisonment rates were recalculated based on the revised adult age and will therefore differ from those quoted in older issues.
Because of rounding, some percentages may not sum to 100. Further, when percentages are being compared, the differences are reported as percentage points. When counts (eg number of incidents) are being compared, the difference is reported as a percentage.