National crime statistics aim to provide comparable data across the states and territories for selected crimes in Australia.
There are two sources of national crime statistics: crimes recorded by police, and crime victimisation surveys. Crimes recorded by police relate to victims of criminal incidents who have become known to police and whose experiences have been recorded by police. These offences may have been reported by a victim, witness or other person, or they may have been detected by police. These statistics do not provide a total picture of crime, as not all crimes come to the attention of police. In addition, care should be taken in interpreting police statistics, as fluctuations in recorded crime may be a reflection of changes in community attitudes to reporting crime, changes and differences in police procedures or resources, or changes and differences in crime recording systems, rather than a change in the incidence of criminal behaviour. Significant events occurring in particular years may also contribute to fluctuations in recorded crime.
A complementary picture of the nature and extent of crime comes from crime victimisation surveys. One of the primary reasons for conducting victimisation surveys is that many victims of crime do not report their experiences to the police, and so are not counted in police data. Victimisation surveys provide more information about the broader community experience of crime, including the volume of crime that is not officially recorded. Crime victimisation surveys are suitable for measuring crimes against individuals (or households) who are aware of and recall the incident and how it happened, and who are willing to relate what they know. These surveys allow crime information to be related to personal and household characteristics, and facilitate the study of patterns of victimisation over time and across crime categories.
Reliable and comprehensive information about certain types of crime such as sexual offences and assaults are difficult to obtain. Problems arise from issues of perception (e.g. whether an incident was one of sexual assault and whether it was a crime) and therefore of self-classification by both the victim and the perpetrator. Under-reporting, hidden reporting, under-recording and hidden recording are also issues that limit attempts to measure particular crimes. In some instances, there may also be reporting of incidents which were not in fact crimes.
Not all types of crime are suitable for measurement by household surveys. No reliable victim-based information can be obtained about crimes where there is no specific victim (e.g. trafficking in narcotics) or where the victim is deceased (e.g. murder). Crimes of which the victim may not be aware cannot be measured effectively; some instances of fraud and many types of attempted crimes fall into this category.