There are many forms of measuring crime, including household surveys or checking hospital or insurance records, but the term "crime statistic" usually refers to figures compiled by the Police and similar law enforcement agencies. However, it is well known that many if not most offences are not reported to the police, and changes in police procedures can have a big impact on how such reported crimes are categorised. This is why public surveys are sometimes conducted to estimate the amount of crime not reported to police and to ascertain levels of victimization. Such surveys are usually more reliable in providing reliable trends over time, but they rarely encompass all crime (eg separate surveys are required to measure retail crime such as shoplifting, as distinct from crime against the public), rarely give local statistics useful for local crime prevention or enforcement, often ignore offences against children, and do not count offenders brought before the criminal justice system.
Crime statistics are gathered and reported by many countries and are of interest to several international organizations, including Interpol and the United Nations. Law enforcement agencies in some countries, such as the FBI in the United States and the Home Office in England & Wales, publish crime indices, which are compilations of statistics for various types of crime.
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Because laws vary between jurisdictions, comparing crime statistics between, and even within, countries can be difficult, sometimes even problematic. Even overall international trends can be difficult to interpret authoritatively. Research using a series of victim surveys in 18 countries of the European Union funded by the European Commission has reported (2005) that the level of crime in Europe has fallen back to the levels of 1990, and notes that levels of common crime have shown declining trends in the U.S.A., Canada, Australia and other industrialised countries as well. The European researchers say a general consensus identifies demographic change as the leading cause for this international trend. However they suggest that "increased use of crime prevention measures may indeed be the common factor behind the near universal decrease in overall levels of crime in the Western world", since decreases have been most pronounced in property crime and less so, if at all, in contact crimes.
Crime indexes are generated to analyze crime statistics. The most common source of such statistics are records reported to the police. Critics do not feel that crime indexes are particularly useful as they only list reported crimes. This makes it difficult to know whether a "10% increase" has any real meaning. An alternative source of data is the victimization survey, usually conducted via annual interviews with households, whose occupants are asked whether they have been the victim of crime. Critics note that surveys are generally unreliable, especially when they involve discussions of criminal activity. Victims surveys however provide more reliable data than police recorded crime for the volume (or common) crimes. Rare but serious crimes such as intentional homicide and armed robberies have frequencies that are too low to be measured accurately by a survey. Considering the seriousness of these crimes and the amount work done on them, police statistics are more accurate here, at least in the developed countries.
For example: Almost all recorded traffic offending is reported either by law enforcement officers or by automatic road safety cameras because there is normally a fine and (profitable) revenue collection process to go through. Yet it is likely that very little traffic offending reported by the public will make its way into official statistics because of the difficulty in following up these reports.
Crime rate is a measure of the rate of occurrence of crimes committed in a given area and time. Most commonly, crime rate is given as the number of crimes committed among a given number of persons.Often, the type of crime is exactly specified. Thus, a crime rate might be given as the number of murders (or rapes, thefts, etc.) per 100,000 persons per year within a city.
Crime rate is a useful statistic for many purposes, such as evaluating the effectiveness of crime prevention measures or the relative safety of a particular city or neighborhood. Crime rate statistics are commonly used by politicians to advocate for or against a policy designed to deal with crime.
The calculation of crime rates uses data that is obtained either from criminal justice systems or from public surveys. Comparisons between the two types of data are problematic, and so are comparisons using the same type of data between different jurisdictions. The United Nations publishes international reports of both Crime Trends and Operations of Criminal Justice A European initiative has resulted in the European sourcebook , an utmost attempt is made to harmonise the criminal justice data for the purpose of international (European) comparison.
Offending that is a breach of the law but for which no punishment exists is often not counted. For example: Suicide, which is technically illegal in most countries, may not be counted as a crime, although attempted suicide and assisting suicide are.
Also traffic offending and other minor offending that might be dealt with by using fines, rather than imprisonment, is often not counted as crime. However separate statistics may be kept for this sort of offending.
Statistics from law enforcement organisations are often used. These statistics are normally readily available and are generally reliable in terms of identifying what crime is being dealt with by law enforcement organisations, as they are gathered by law enforcement officers in the course of their duties and are often extracted directly from law enforcement computer systems. However, these statistics often tend to reflect the productivity and law enforcement activities of the officers concerned and may bear little relationship to the actual amount of crime, as officers can only record crime that comes to their attention and might not record a matter as a crime if the matter is considered minor and is not perceived as a crime by the officer concerned. The statistics may also be biased because of routine actions and pragmatic decisions that law enforcement officers make in the field. For example, when faced with a domestic violence dispute between a couple, a law enforcement officer may decide it is far less trouble to arrest the male party to the dispute, because the female may have children to care for, despite both parties being equally culpable for the dispute. This sort of pragmatic decisionmaking would mean the statistics had a gender bias because of what officers did to resolve such disputes.
Researchers also conduct population surveys in order to identify victims of crime and their experiences. These surveys are often known as victimisation surveys as they seek to identify the victims of crime, especially for crime that is not reported to a law enforcement officer. Victimisation surveys also suffer from problems as they are perception surveys. In these surveys people are being asked if they are victims of crime, without needing to provide any supporting evidence. In these surveys it is the participant's perception, or opinion, that a crime occurred, or even their understanding about what constitutes a crime that is being measured. As a consequence victimisation surveys can also exhibit a subjective bias. Also, differing methodologies may make comparisons with other surveys difficult.
One way in which victimisation surveys are useful is that they show some types of crime are well reported to law enforcement officials, while other types of crime are under reported. These surveys also give insights as to why crime is reported, or not. The surveys show that the need to make an insurance claim, seek medical assistance, and the seriousness of an offence tend to increase the level of reporting, while the inconvenience of reporting, the involvement of intimate parters and the nature of the offending tend to decrease reporting. This allows degrees of confidence to be assigned to various crime statistics. For example: Motor vehicle thefts are generally well reported because the victim may need to make the report for an insurance claim, while domestic violence, domestic child abuse and sexual offences are frequently significantly under-reported because of the intimate relationships involved, embarrassment and other factors that make it difficult for the victim to make a report.
Attempts to use victimisation surveys from different countries for international comparison had failed in the past. A standardised survey project called the International Crime Victims Survey has been set up to specifically to ínsure international comparison. The project started in 1989 and preparations for its 6th round of surveys in the spring of 2009 are taken. Results from this project have been breefly discussed earlier in this article.
Classification systems attempt to overcome these problems, although different jurisdictions perform this classification in different ways. Some classification systems concentrate on specific indicator crimes, such as murder, robbery, burglary and vehicle thefts. Other systems, such as the Australian Standard Offence Classification(ASOC) attempt to be more comprehensive.
The International Crime victims Survey has been done is over 70 countries to date and has become the 'de facto' standard for defining common crimes. Complete list of countries participating and the 11 defined crimes can be found at the project web site
More complex measures involve measuring the numbers of discrete victims and offenders as well as repeat victimisation rates and recidivism. Repeat victimisation involves measuring how often the same victim is subjected to a repeat occurrence of an offence, often by the same offender. Repetition rate measures are often used to assess the effectiveness of interventions.
Because crime is a social issue, comparisons of crime between places or years are normally performed on some sort of population basis.