Abstract
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BACKGROUND FOR THE REVIEW
Closed circuit television (CCTV) surveillance cameras serve many functions and are used in both public and private settings. The prevention of personal and property crime is among the primary objectives in public space. As an intervention targeted at crime, CCTV is a type of situational crime prevention (Clarke, 1995). According to Clarke and Homel's (1997) classification of situational crime prevention, CCTV is viewed as a technique of “formal surveillance.” In this regard, CCTV cameras are seen to enhance or take the place of security personnel.
It is argued that CCTV (especially if well publicized) may prevent crime because potential offenders are deterred by their increased subjective probability of detection. Also, CCTV may increase the true probability of detection, may increase pedestrian usage of places and hence further increase the subjective probability, may encourage potential victims to take security precautions, and may direct police and security personnel to intervene to prevent crime (Armitage, Smyth, and Pease, 1999, pp. 226–227). Another possibility is that CCTV could signal improvements in the area and hence increase community pride, community cohesion, and informal social control.
CCTV could also cause crime to increase. For example, it could give potential victims a false sense of security and make them more vulnerable because they relax their vigilance or stop taking precautions, such as walking in groups at night and not wearing expensive jewelry. It may encourage increased reporting of crimes to the police and increased recording of crimes by the police. CCTV may also cause crime to be displaced to other locations, times, or victims.
In recent years, there has been a tremendous growth in the use of CCTV to prevent crime in public space, especially in Britain (Norris and Armstrong, 1999) and, to a much lesser extent, in the U.S. (Nieto, 1997). In Britain, CCTV is the single most heavily funded non-criminal justice crime prevention measure. Over the three-year period of 1999 through 2001, the British government made available £170 million (approximately $250 million) for CCTV schemes in town and city centers, car parks, and other high crime areas (Home Office Policing and Reducing Crime Unit, 2001, p. 8). In previous years, CCTV accounted for more than three-quarters of total spending on crime prevention by the UK Home Office (Koch, 1998, p. 49). According to Armitage (2002), the number of cameras has increased from 100 in 1990 to 400 in 1994, 5,200 in 1997, and 40,000 in 2002.
During this time there has been much debate about the effectiveness of CCTV to prevent crime and, hence, on the wisdom of spending such large sums of money. A key issue is how far funding for CCTV in Britain has been based on high quality scientific evidence demonstrating its efficacy in preventing crime. There is concern that this funding has been based partly on a handful of apparently successful schemes that were usually evaluated using simple one group (no control group) before-after designs, done with varying degrees of competence (Armitage, Smyth, and Pease, 1999, p. 226), and done with varying degrees of professional independence from the Home Office (Ditton and Short, 1999, p. 202). Recent reviews that have examined the effectiveness of CCTV against crime (Eck, 2002; Nieto, 1997; Phillips, 1999; Poyner, 1993) have also noted the need for high quality, independent evaluation research.
OBJECTIVES OF THE REVIEW
The main objective of this review is to assess the available research evidence on the effects of CCTV surveillance cameras on crime in public space. In addition to assessing the overall impact of CCTV on crime, this review will also investigate in which settings (e.g., city centers, car parks), against which crimes, and under what conditions it is most effective.
METHODOLOGY
Criteria for Inclusion and Exclusion of Studies in the Review
In selecting studies for inclusion in the review, the following criteria will be used:
CCTV is the focus of the intervention. For studies involving one or more other interventions, only those studies in which CCTV was the main intervention will be included. The determination of the main intervention will be based on the study author identifying it as such or, if the author does not do this, the importance of CCTV relative to the other interventions. There is an outcome measure of crime. Where applicable, crime outcome data will be reported separately for two main categories: official records (police reports) and unofficial measures (victim survey or self-report survey). The evaluation design is of adequate or good methodological quality, with the minimum design involving before-and-after measures of crime in experimental and control areas. The unit of interest is areas. There is at least one experimental area and one reasonably comparable control area. The total number of crimes in each area before the intervention is at least 20. The main measure of effect size (see below) is based on changes in numbers of crimes between the before and after time periods. A minimum of 20 crimes in the before period was set because it was considered that a measure of change based on an
Studies that do not meet one or more of the above criteria will be excluded from the review. Both reviewers will independently screen the full text of studies and recommend whether to include them in the review. Disagreements will be resolved upon discussion and if no resolution is achieved, the study will not be included in the review. Each excluded study will be listed along with a reason for its exclusion.
Search Strategy for Identification of Relevant Studies
The following four search strategies will be carried out to identify studies meeting the criteria for inclusion in the review:
Searches of on-line data bases (see below). Searches of literature reviews on the effectiveness of CCTV in preventing crime. Searches of bibliographies of CCTV studies. Contacts with leading researchers.
Both published and unpublished studies will be considered. Searches will be international in scope and will not be limited to the English language.
The following data bases will be searched:
Criminal Justice Abstracts. National Criminal Justice Reference Service (NCJRS) Abstracts. Sociological Abstracts. Social Science Abstracts (SocialSciAbs). Educational Resources Information Clearinghouse (ERIC). Government Publications Office Monthly Catalog (GPO Monthly). Psychology Information (PsychInfo). Public Affairs Information Service (PAIS) International. Dissertation Abstracts. Australian Criminology Database (CINCH).
The following terms will be used to search the data bases:
Closed circuit television. CCTV. Cameras. Social control. Surveillance. Formal surveillance.
Where applicable, “crime” will be added to each of the above terms (e.g., CCTV and crime) to narrow the search parameters.
Description of Methods Used in Primary Research
The following descriptions are of a few of the CCTV studies that have been judged to meet the inclusion criteria for the review. They are drawn from different settings in which CCTV surveillance cameras have been implemented to reduce crime.
Skinns (1998, p. 176) evaluated a “multi-agency, police-led, town centre system, consisting of 63 cameras located in the commercial centre, multi-storey car parks and main town centre arterial roads” in Doncaster, England. (The program is categorized as a city/town center scheme, as opposed to a car parks scheme, because the main focus of the intervention was the town center.) Another intervention was used: “help points” were established within the experimental area to aid the public in contacting the main CCTV control room. The experimental area included all or parts of streets in vision of the cameras. The control area includes commercial areas of four non-adjacent townships. Twenty-four months after the start of the program, total police-recorded crime had reduced in the experimental area by 21.3%, but it had increased in the control area by 11.9%. Based on police records and interviews with young offenders in adjacent areas, where crime decreased, the author found no evidence that total crimes were displaced from the experimental area to the control area.
In the Montreal subway, the “Metro,” CCTV cameras were installed in 13 stations (approximately ten cameras per station) over the course of 18 months in the early 1990s. Fifty-two stations served as the control group. Grandmaison and Tremblay (1997) evaluated the program after 18 months of operation, and statistical analyses were carried out to control for past crime trends in the experimental and control stations. The authors found an equal reduction in (police-recorded) crime in both the experimental and control subway stations: −20.0% and −18.3%, respectively. The measure of total crime included robbery, assault, purse snatching, other theft and fraud, vandalism, and other offenses. From 18 months before the start of the intervention to 18 months afterwards, all categories of crimes were down in the experimental stations, while all categories except assault decreased in the control stations. The authors did not investigate the possibility of displacement or diffusion of benefits.
Poyner (1991) evaluated a multi-component scheme at the University of Surrey in Guildford, England, in which both the experimental and control parking lots (one in each condition) received up-graded lighting and foliage was cut back, but only the experimental parking lot received CCTV. Ten months after the program started, the author found that thefts from vehicles were substantially reduced in both the experimental and control parking lots. In the experimental site, the monthly average of incidents declined by almost three-quarters (73.3%; from 3.0 to 0.8), while in the control site, they were almost eliminated (a drop of 93.8%; from 1.6 to 0.1). While the author concluded that there was evidence of diffusion of benefits (from the experimental area to the control area), a non-adjacent control area, which is needed to test for geographical or territorial displacement or diffusion of benefits (see below), was not used.
Criteria for Determination of Independent Findings
In the case of studies that measure the impact of CCTV programs on crime and other outcomes (e.g., ridership on public transport, commercial activity in city centers), only crime outcomes will be examined as part of the review.
In the case of studies that measure the impact of CCTV programs on crime at multiple points in time, similar time periods before and after (e.g., 12 months) will be compared (as far as possible). If reported, long-term and short-term effects could be compared.
Details of Study Coding Categories
The following characteristics of the included studies will be retrieved and retained for examination as potential moderators of study outcomes:
Author, publication date, and location. The authors and dates of the most relevant evaluation reports and the location of the program will be identified. Context of intervention. This is defined as the physical setting in which the CCTV intervention took place. CCTV cameras. The number of CCTV surveillance cameras in operation and any special technological features of the cameras (e.g., infrared, pan or zoom capability), as well as the use of any “dummy” cameras (not real or not operational), will be identified. Coverage. The coverage area of CCTV surveillance cameras will be identified. Monitoring. How (i.e., active or passive) and by whom (e.g., police, private security) the CCTV cameras are monitored will be identified. Duration of intervention. The length of time the program was in operation will be identified. Sample size. The number and any special features of the experimental and control areas will be identified. Other interventions. Interventions other than CCTV that were employed at the time of the program will be identified. For example, many of the CCTV schemes employed notices or signs to publicize the use of cameras in an area, and it may be possible to compare the results of those CCTV schemes with and without notices. Outcome measure of interest and data source. Crime is the outcome measure of interest to the review. The specific crime types and the data source of the outcome measure (e.g., police records, victim survey) will be identified. Research design. The type of evaluation design used to assess the program's impact on crime will be identified. If matching or other statistical analysis techniques are used as part of the evaluation of program effects, these will be noted. Before-after time period. The before and after time periods of the evaluation will be identified.
As noted above, the outcome measure of interest to this review is crime, specifically, property (e.g., burglary, theft of vehicles) and violent (e.g., assault, robbery) crimes. In summarizing results, the focus will be on the outcome of interest to this review and comparisons between experimental and control areas (see below for more details). Results will be reported for total crime and, where possible, property and violent crime categories and specific crimes.
The review will also report on studies of displacement of crime and diffusion of crime prevention benefits. Displacement is often defined as the unintended increase in crimes in other locations following from the introduction of a crime reduction scheme. Six different forms of displacement have been identified: temporal (change in time), tactical (change in method), target (change in victim), territorial (change in place), functional (change in type of crime), and perpetrator (Reppetto, 1976; Barr and Pease, 1990). Diffusion of benefits is often defined as the unintended decrease in crimes in other locations following from a crime reduction scheme, or the “complete reverse” of displacement (Clarke and Weisburd, 1994). In order to investigate territorial displacement and diffusion of benefits, the minimum design involves one experimental area, one adjacent area, and one non-adjacent control area. If crime decreased in the experimental area, increased in the adjacent area, and stayed constant in the control area, this might be evidence of displacement. If crime decreased in the experimental and adjacent areas and stayed constant or increased in the control area, this might be evidence of diffusion of benefits.
Where information is provided about the number of targets in an area (e.g., number of pedestrians, number of houses or cars), we will analyze it to study crime rates. One methodological problem is that CCTV may cause more people to use the streets and hence cause more targets and opportunities for crime.
Coding reliability will be established by the use of a descriptive checklist of all moderating variables of interest to the review. Coding reliability will be monitored by the co-reviewer who will review a random sample of studies.
Statistical Procedures and Conventions
We plan to carry out a meta-analysis in order to estimate the average effect size in evaluations of the effects of CCTV on crime. In order to complete a meta-analysis, a comparable effect size is needed in each evaluation, together with its variance. This has to be based on the number of crimes in experimental and control areas in time periods (most commonly of 12 months) before and after the intervention, because this is the only information that is regularly provided in all the evaluations.
While studies based on police records can present time series data, studies based on victim surveys usually have data only for one time period before the intervention and one time period after. Because of the problem that the intervention may cause more reporting to police and recording by police, it is important to analyze both police and victim survey data.
We will use the odds ratio (OR) to measure the effect size. The OR is calculated from the following table:
where a, b, c, d are numbers of crimes
OR = ad/bc
In calculating an average effect size for all or a subset of the studies, it will be inversely weighted according to the variance of each study, as specified in Lipsey and Wilson (2001). Also, in calculating an average effect size for all or a subset of the studies, statistical tests will be carried out to assess if the individual effect sizes were randomly distributed around the average effect size (or if there is heterogeneity). Moderators that predict effect sizes will be investigated (where available).
The OR is meaningful because it specifies the relative change in crime in a control area compared with an experimental area. For example, if the weighted average OR was 1.25 this would show that crime increased by 25% more in control areas than in experimental areas, or conversely that crime decreased 20% more in experimental areas than in control areas (using the inverse of the OR = 1/1.25).
The variance of OR is calculated from the variance of LOR (the natural logarithm of OR). The usual calculation of this is as follows:
We have received statistical advice from Dr. Patricia Altham of the Centre for Mathematical Sciences, University of Cambridge, that this estimate of the variance needs to be corrected in two ways:
It needs to be decreased to take account of positive dependence (the correlation r) between the number of before crimes and the number of after crimes experienced by victims. It needs to be increased to take account of overdispersion Φ (the ratio of the variance to the mean of the number of crimes experienced by victims).
V1 (LOR) = Φ (V - C)
Where V1 (LOR) = corrected variance of LOR
and C = 2r[1/√ab + 1/√cd]
The values of r and Φ are not fully known for these evaluations. However, based on the best estimates of these values in the Dudley study of improved lighting (r = .3 – .4; Φ = 3 – 5), the corrected variance would be about 2–3 times greater than the original variance. We therefore plan to recalculate our meta-analysis assuming that every SE(LOR) was (i) 50% greater, and (ii) 80% greater, to give some idea of the likely range of results. This is equivalent to a range of variances between 2.25 times and 3.24 times the original variances.
Treatment of Qualitative Research
Qualitative research will not be included in this systematic review and meta-analysis.
TIME FRAME
PLANS FOR UPDATING THE REVIEW
The review will be updated every two years. The two reviewers will be responsible for the next update and all updates thereafter.
ACKNOWLEDGMENTS
No other individuals or organizations contributed to the preparation of this protocol.
STATEMENT CONCERNING CONFLICT OF INTEREST
There is no conflict of interest.