Police programmes that seek to increase community connectedness for reducing violent extremism behaviour, attitudes and beliefs

Abstract Background Police can play a role in tackling violent extremism through disrupting terrorist plots and by working with communities to identify individuals at risk of radicalisation. Police programmes to tackle violent extremism can involve a range of approaches and partnerships. One approach includes efforts to improve community connectedness by working to address social isolation, belonging, economic opportunities and norms and values that may lead people to endorse or support violent extremist causes and groups. The assumption is that the risk of an individual being radicalised in the community can be reduced when police work in pothe international legal ordersitive ways with community members and groups to mobilise and support activities that help generate a sense of belonging and trust. Police programmes that build a sense of belonging and trust may help ensure individuals are not influenced by activities that violent extremists use to attract support for their cause. Objectives The review aimed to systematically examine whether or not police programmes that seek to promote community connectedness are effective in reducing violent extremist behaviours, attitudes and beliefs. The review also sought to identify whether effectiveness varied by the intervention type and location. Search Methods Using terrorism‐related terms, we searched the Global Policing Database to identify eligible published and unpublished evaluations between January 2002 and December 2018. We supplemented this with comprehensive searches of relevant terrorism and counter‐terrorism websites and research repositories, reference harvesting of eligible and topic‐relevant studies, forward citation searches of eligible studies, hand‐searches of leading journals and consultations with experts. Selection Criteria Eligible studies needed to include an initiative that involved the police, either through police initiation, development, leadership or where the police were receivers of the programme (such as a training programme) or where the police delivered or implemented the intervention. The initiative also needed to be some kind of a strategy, technique, approach, activity, campaign, training, programme, directive or funding/organisational change that involved police in some way to promote community connectedness. Community connectedness was defined as being community consultation, partnership or collaboration with citizens and/or organisational entities. Eligible outcomes included violent extremism, along with radicalisation and disengagement which are considered to be attitudinal and belief‐based components of violent extremism. These outcomes could be measured via self‐report instruments, interviews, observations and/or official data. To be included, studies could utilise individuals, micro‐ or macroplaces as the participants. Finally, studies needed to provide a quantitative impact evaluation that utilised a randomised or quasi‐experimental design with a comparison group that either did not receive the intervention, or that received “business‐as‐usual” policing, no intervention or an alternative intervention. Data Collection and Analysis The systematic search identified 2,273 records (after duplicate removal). After systematic screening across two stages (title/abstract and full‐text), just one study (reported in two documents) met the review eligibility criteria. Standardised mean differences (SMD) were used to estimate intervention effects for this single study and risk of bias was assessed using the Cochrane Risk of Bias in Non‐Randomised Studies‐Interventions tool (ROBINS‐I). Results The single eligible study (n = 191) was a quasi‐experimental evaluation of the Muslim‐led intervention—World Organisation for Resource Development Education (WORDE)—conducted in the United States in 2015. The intervention comprised three components: community education, enhancing agency networks and multicultural volunteerism activities. Self‐report data were collected from youth and adults who were civically engaged, sensitised to issues of violent extremism and who had existing cooperative relationships with law enforcement and social services. The comparison group comprised matched participants who had not engaged with the WORDE programme. The outcomes most closely aligned with conceptual definitions of deradicalization, specifically levels of acceptance and/or engagement with cultural and religious differences or pluralistic views and modification of group or personal identity. Based on single survey items, the SMD ranged from small to medium in favour of the treatment group aside from one item which favoured the comparison group (“I make friends with people from other races”, SMD = −0.51, 95% CI: −0.82, −0.19). However, of the nine SMDs calculated, six had confidence intervals including zero. These effects should be interpreted with caution due to the study's overall serious risk of bias. It is important to note that it is not explicitly clear whether the evaluation participants in the treatment group were all directly exposed to the two intervention components that involved police. Hence, these evaluation outcomes may not be direct measures of how effect police were at countering violent extremism by promoting community connectiveness. Conclusions The aim of this systematic review was to examine whether or not police programmes that seek to promote community connectedness are effective in reducing violent extremist behaviours, attitudes and beliefs. There is insufficient evidence available to ascertain whether such interventions achieve these outcomes. This finding is the result of the fact that interventions that have been evaluated tend to be characterised by evaluation designs that do not adopt experimental or quasi‐experimental approaches or use outcomes that are outside of scope for this review. While the volume of studies identified provide support for the assertion that police can play a role in tackling violent extremism by participating in, and implementing, programmes that promote community connectedness, it is unclear at this time if such approaches work in reducing violent extremism. Whilst we conclude that investment needs to be made in more robust methods of evaluation to test for programme effectiveness, we acknowledge that conducting evaluation and research in the area of counter‐terrorism/violent extremism is challenging.

evaluation designs that do not adopt experimental or quasi-experimental approaches or use outcomes that are outside of scope for this review. While the volume of studies identified provide support for the assertion that police can play a role in tackling violent extremism by participating in, and implementing, programmes that promote community connectedness, it is unclear at this time if such approaches work in reducing violent extremism. Whilst we conclude that investment needs to be made in more robust methods of evaluation to test for programme effectiveness, we acknowledge that conducting evaluation and research in the area of counter-terrorism/violent extremism is challenging.
1 | PLAIN LANGUAGE SUMMARY 1.1 | There is limited evidence of how police programmes to generate community connectedness affect violent extremist behaviours, attitudes and beliefs Police programmes to generate community connectedness are assumed to help reduce risk factors that lead individuals to radicalise to violent extremism. There is no robust body of evaluation evidence to verify this claim. This lack of evidence is because programme funders have not sufficiently invested in impact evaluations of policing programmes that aim to counter violent extremism by promoting community connectedness.

| What is this review about?
Community connectedness and efforts to engage communities may help to mitigate the risk of individuals radicalising to violent extremism. Police, under some circumstances, can play a key role in programmes aimed at tackling violent extremism. This includes working with communities and other agencies to tackle social isolation, economic opportunity and norms and beliefs that lead individuals and groups to radicalise and support extremist causes.
This review looked at whether or not strategies involving police in the initiation, development or implementation of programmes aimed at community connectedness had an impact on reducing violent extremist beliefs and behaviours.

| What studies are included?
The review includes studies that evaluated programmes aimed at countering violent extremism by promoting community connectedness. The interventions included in the review needed to have a police focus, where the intervention involved police as the receivers of an intervention and/or partners in the development, initiation and implementation of a programme. The intervention could be focused on individuals, places (e.g., schools), neighbourhoods or larger geographical locations.
Although the systematic search captured 2,273 potential studies, only one study met the review inclusion criteria. This study was conducted in 2015 in the USA.

| What are the findings of this review?
The one included study was a Muslim community-led initiative involving police that aimed to counter violent extremism through a community-based education and awareness programme. The programme aimed to improve referral networks for agencies/ third parties to help assist individuals identified as at-risk of radicalisation.
Evidence from this study showed mixed small-to-medium effects on self-reported deradicalization measures in favour of the treatment group. Eight out of nine calculated effect sizes favoured the intervention, though six of these were statistically insignificant. One survey item favoured the comparison group: "I make friends with people from other races". However, these results need to be interpreted with caution due to the study limitations.
Given the low number of studies identified, the authors have also provided a summary of a small sample of studies reporting on interventions that aligned with the review topic but did not meet the inclusion criteria due to weak evaluation designs. These studies illustrate a range of approaches being used by the police, such as recreation and sports activities, and community education and engagement around countering violent extremism and related topics.
What is the aim of this review? This Campbell systematic review examines whether police programmes aiming to improve community connectedness have an effect on violent extremist behaviour, attitudes and beliefs. It summarises evidence from one study that met the inclusion criteria and references others that describe types of interventions, but that have not been rigorously evaluated. MAZEROLLE ET AL. | 3 of 48 1.5 | What do the findings of this review mean?
There is currently insufficient evidence to establish whether police programmes aimed at countering violent extremism by promoting community connectedness are effective. Although the evidence identified by this review shows that such programmes are being implemented, they have not yet been rigorously evaluated. Future research should aim to rigorously evaluate such initiatives.
1.6 | How up-to-date is this review?
The review authors searched for studies up to December 2018.
2 | BACKGROUND 2.1 | The problem, condition or issue Community engagement and connectedness are identified as potential mitigating factors for those at risk of engaging in violent extremism (Cherney & Hartley, 2017). A focus on inclusion, social connectedness and positive cultural norms is essential for prevention efforts designed to build inclusive communities and weaken the influence of extremist messages and recruiters (Grossman, Peucker, Smith, & Dellal, 2016;Van Den Bos, 2018;Schanzer, Kurzman, Toliver, & Miller, 2016). Growing research suggests that cohesive communities are resilient against violent extremist influences; for example, it is argued that a greater sense of belonging and acceptance can reduce extremist behaviour, attitudes and beliefs (Cherney, Bell, Leslie, Cherney, & Mazerolle, 2018;Grossman et al., 2016;Van Den Bos, 2018).
As frontline practitioners, police are well placed to promote social inclusion and social connectedness, and thereby preventing violent extremism. In the general policing literature, when police engagement with the public is undertaken in an inclusive and fair manner, police can be instrumental in fostering a deep understanding of the local communities they police, creating opportunities for improving community relations (Gill, Weisburd, Telep, Vitter, & Bennett, 2014). Strong police-community relations are likely an important foundation for police being in a position to identify individuals who might be at risk of radicalisation and violent extremism and then work with community leaders to counter the influence of a variety of different types of violent extremist groups including those from the far right, the far left, environmental extremism, political and religious extremism groups. Police can, therefore, be key agents in promoting community connectedness, working with community members to build trust, minimise social distancing-particularly amongst culturally diverse communities-and strengthen a sense of belonging by showing that they have the interests of the community at heart (Cherney & Hartley, 2017;Murray, Mueller-Johnson, & Sherman, 2015).
Increasingly, police are working with a range of different agencies, together actively engaging the community to reduce social isolation, improve economic opportunity and aim to create social and cultural norms that prevent violent extremism ).
Yet it is unclear whether or not the range of police initiatives that foster community connectedness are able to reduce violent extremism. Thus, it is essential to understand the effectiveness of policing programmes aimed at promoting community connectedness and their impact on reducing violent extremism.

| The intervention
This review aimed to include any policing intervention that aims to promote community connectedness, which was defined by the presence of two components. First, the intervention must have had a policing focus, defined as some kind of a strategy, technique, approach, activity, campaign, training, programme, directive or funding/ organisational change that involves police in some way (other agencies or organisations can be involved; Higginson, Eggins, Mazerolle, & Stanko, 2015). Police involvement was broadly defined as follows.
• Police initiation, development or leadership of the intervention.
• Police are receivers of the intervention (such as being the receivers of training programmes or on the receiving end of initiatives aimed to engage police in the activities of another agency) or the intervention is related, focused or targeted to police practices.
• Delivery or implementation of the intervention by police.
Second, the policing intervention must have aimed to promote community connectedness. For the purposes of this review, we defined the promotion of community connectedness to mean an intention to increase prosocial linkages or prosocial ties between either community members themselves, community members and police, or community members and people in businesses, houses of worship, schools or any other community-based organisation. Other terminology that may be used to represent connectedness in the literature includes (Thomas, 2019) the following.
• Promotion of common values, norms and/or reciprocity.
• Promotion of social networks, collective efficacy, social cohesion or social capital.
• Promotion of shared problem solving or citizen engagement.
We anticipated that policing interventions aiming to promote community connectedness would likely overlap with initiatives labelled community policing (see Gill et al., 2014) or other policing approaches that often aim to enhance community connectedness (e.g., neighbourhood policing or legitimacy approaches). However, we specifically defined a policing intervention that aims to promote community connectedness as being characterised by community consultation, partnership or collaboration with citizens and/or organisational entities.
Specific strategies may have included the following: • community meetings or forums; • developing partnerships with specific organisations (Fox, 2012); • police liaison programmes involving community members ; • police work with community leaders to enhance personal skills (e.g., self-identity, self-awareness and resilience), employment skills (e.g., teamwork and self-awareness), or leadership skills (Thomas, 2019); • routine police work (such as beat policing, foot patrols, community intelligence initiatives) that explicitly seek to promote community connectedness; • specific initiatives-such as neighbourhood policing teams-that seek to promote community connectedness; or • police legitimacy enhancing programmes that seek to promote a sense of belonging and inclusion within local communities.

| How the intervention might work
Police programmes that seek to reduce violent extremist behaviours and beliefs through improving community connectedness aim to generate an impact by promoting an increased sense of prosocial belonging and inclusion amongst at-risk groups. This causal pathway is underpinned by key perspectives in the literature that argue how people are treated by institutional authorities, such as police, has an impact on their sense of identity and belonging by making them feel accepted by broader society .
Social identity theory and the group value model (see Tyler & Lind, 1992) demonstrates that the ways that police engage with citizens will differentially affect the way that people perceive the police and thereby their willingness to comply with directives (see also Bradford, Murphy, & Jackson, 2014;Huo, Smith, Tyler, & Lind, 1996). Some research shows that procedural fairness is more important for those on the margins (De Cremer & Sedikides, 2005;Murphy, 2013). Other research finds that the procedural justice, social identity and legitimacy pathway is found amongst both those with high and low group identifications (Bradford et al. 2014). Outcomes of police initiatives that build a sense of belonging and inclusion are, therefore, assumed to act as protective factors against radicalisation by ensuring individuals are not influenced by the messaging and grievance narratives that violent extremists use to attract support. We acknowledge, however, Nagin and Telep's (2017) review of the evidence challenging the causal relationship between perceptions of procedurally just treatment of citizens by agents of the criminal justice system and perceptions of police legitimacy. These authors conclude that perceptions of procedurally just treatment are associated with perceptions of police legitimacy and legal compliance (see also Donner, Maskaly, Fridell, & Jennings, 2015;Jackson, Hough, Bradford, & Kuha, 2015;Tyler 2004;Tyler, Goff, & MacCoun, 2015), yet that these associations are not necessarily reflective of causal connections.
As it relates specifically to tackling violent extremism, the quality of police engagement with different types of communitiessuch as the Muslim community-has been shown to influence the degree to which people are willing to partner with police to tackle terrorism . Police are seen as legitimate authorities and representatives of the state and when they work with community groups. This helps builds police legitimacy and has a spill over effect on people's sense of belonging and inclusion.

| Why it is important to do the review
Community engagement approaches have become a key component of police counter-terrorism efforts (Cherney & Hartley, 2017). These strategies have emphasised community engagement and outreach to identify potential violent extremism threats. This has involved police programmes that aim to promote collaborative problem solving between police and community members to tackle radicalisation, such as through identifying youth at risk of radicalising to violent extremism (Cherney, 2018). For example, following 9/11, police units in Australia, the United Kingdom, the United States, and Canada were established to undertake outreach with particular community groups (e.g., Muslim communities), with the aim of tackling violence extremism by enhancing relations and connectedness between police and these communities, and also between community members (Cherney, 2018;Ramirez, 2012). However, to date, there has been no systematic synthesis of the evaluation evidence for these policing approaches and their impact on violent extremism. 1 Therefore, the current review is necessary to ascertain whether policing interventions that seek to promote community connectedness are effective for reducing violent extremism behaviour, attitudes and beliefs. In addition, the results from this review can be used to inform future decision making relating to both the design and evaluation of police programmes by identifying gaps in the evidence-base and level of investment needed in evaluation of primary studies.

| OBJECTIVES
The primary objective of this review was to answer the question: how effective are police programmes that seek to increase community connectedness for reducing violent extremism attitudes, beliefs and behaviours? A secondary objective was to also examine whether the effectiveness of these interventions vary by the following factors: geographical location, target population and type of policing strategy used to promote connectedness. This review includes quantitative impact evaluations that utilise a randomised experimental (e.g., RCTs) or a quasi-experimental design with a comparison group that does not receive the intervention. We will include studies where the comparison group receives "businessas-usual" policing, no intervention or an alternative intervention (treatment-treatment designs).
Although not as robust as RCTs, "strong" quasi-experiments can be used to provide causal inference when there are elements of the design that aim to minimise threats to internal validity (see Farrington, 2003;Shadish, Cook, & Campbell, 2002). Minimising threats to internal validity can include: controlling case assignment to treatment and control groups (regression discontinuity), matching characteristics of the treatment and control groups (matched control), statistically accounting for differences between the treatment and control groups (designs using multiple regression analysis) or providing a difference-in-difference analysis (parallel cohorts with pre-test and post-test measures). Therefore, we included the following "strong" quasi-experimental designs in this review: • cross-over designs; • regression discontinuity designs; • designs using multivariate controls (e.g., multiple regression); • matched control group designs with or without preintervention baseline measures (propensity or statistically matched); • unmatched control group designs without preintervention measures where the control group has face validity; • unmatched control group designs with pre-post intervention measures that allow for difference-in-difference analysis; • short interrupted time-series designs with control group 2 (less than 25 pre-and 25 postintervention observations; Glass, 1997); • long interrupted time-series designs with or without a control group (≥25 pre-and postintervention observations; Glass, 1997).
Weaker quasi-experimental designs can be used to demonstrate the magnitude of the relationship between an intervention and an outcome.
However, we excluded the following weaker quasi-experimental designs due to their limitations in establishing causality.
• Raw unadjusted correlational designs where the variation in the level of the intervention is compared to the variation in the level of the outcome.
• Single group designs with pre-and postintervention measures.

| Types of participants
We included studies that considered the impact of community connectedness policing interventions on the following population subjects: 1. individuals of any age, gender or ethnicity; 2. microplaces (e.g., street corners, buildings, police beats, street segments); 3. macroplaces (e.g., neighbourhoods or larger geographies).
This review aimed to include programmes focused on individuals and groups identified as at-risk violent extremism due to beliefs and or associations, as well as those who have acted on those beliefs. This was to ensure we captured police programmes tackling different levels of violent extremism. However, to be included in the review, participants did not need to be classified atrisk or have displayed extremist behaviours. We placed no limits on the geographical region reported in the study. Specifically, we included studies conducted in high-, low-and middle-income countries in the review.

| Types of interventions
This review aimed to include any policing intervention that aims to promote community connectedness. Specifically, to be included in the review, a study must have met the following two intervention criteria.
1. Report on a policing intervention, defined as some kind of a strategy, technique, approach, activity, campaign, training, programme, directive or funding/organisational change that involves police in some way (other agencies or organisations can be involved; Higginson et al., 2015). Police involvement is broadly defined as: • police initiation, development or leadership; • police are receivers of the intervention or the intervention is related, focused or targeted to police practices; or • delivery or implementation of the intervention by police.
2. Report on a policing intervention that aims to promote community connectedness. For the purposes of this review, we define the promotion of community connectedness to mean an intention to increase linkages or ties between either the community members themselves, or community members and police. Other terminology that may be used to represent connectedness in the literature includes (Thomas, 2019): • promotion of common values, norms and/or reciprocity; • promotion of social networks, collective efficacy, social cohesion and/or social capital; or • promotion of shared problem solving or citizen engagement. 2 We will include all short interrupted time series designs with control group, as long as the design includes a minimum of one preintervention observation for each of the treatment and comparison groups. This approach is consistent with the inclusion of unmatched control group designs with pre-post intervention measures. For studies with extremely short preintervention time series (less than 4 pre-and 4 post-intervention observations), the data will be collapsed and treated as pre-post averages, rather than as true time series data.
We anticipated that policing interventions aiming to promote community connectedness will include, more generally, community consultation, partnership or collaboration with citizens and/or organisational entities. Specific strategies may include (but are not limited to) the following.
• Community meetings or forums.
• Police liaison programmes involving community members .
• Police work with community leaders to enhance personal skills (e.g., self-identity, self-awareness and resilience), employment skills (e.g., teamwork and self-awareness) or leadership skills (Thomas, 2019).

Primary outcomes
Terrorism is one outcome of violent extremism, which constitutes both a cognitive and behavioural component. In the literature, a distinction is made between radicalisation as constituting beliefs, while violent extremism is the behavioural outcome of those beliefs.
Hence, this review aimed to include studies where the measured outcome was violent extremist attitudes, beliefs and behaviour. For the purposes of this review, violent extremism was defined as "advocating, engaging in, preparing, or otherwise supporting ideologically motivated or justified violence to further social, economic, and political objectives" (Barker, 2015;Horgan, 2009;Khalil and Zeuthen 2016;United States Agency for International Development, 2016).
It is important to note that violent extremism is defined and captured differently across countries (e.g., Barker, 2015;Government Offices of Sweden, 2011;Lowe, 2017;Norwegian Ministry of Justice and Public Security, 2014;Public Safety Canada, 2018). In order to capture research on violent extremism across global contexts, we used either the study authors' definitions of outcomes or, in the case of ambiguity, consulted definitions used in each country's associated terrorism legislation or policies to guide the inclusion of studies with potentially eligible outcomes.
The review also aimed to include studies where the outcome was disengagement and/or deradicalization, which are often encompassed within conceptualisations of violent extremism (Klausen, Campion, Needle, Nguyen, & Libretti, 2016). Disengagement generally captures the behavioural aspect of extremism and refers to reducing or ceasing physical involvement in violent or radical activities (Horgan, 2009). In contrast, deradicalization is defined as the psychological shift in attitudes or beliefs (Horgan & Braddock, 2010). We included outcome data that were measured through selfreport instruments, interviews, observations and/or official data (e.g., arrests or convictions). Some examples of how violent extremism attitudes, beliefs and behaviour can be measured include the following.
• Official data taken from the Profiles of Radicalized Individuals in the United States (PIRUS), 3 which includes: active participation in operational plots intending to cause causalities (e.g., gathering weapons, choosing targets), recruiting individuals to an official or unofficial extremist group and providing material/financial support to extremist organisations (START, 2018).
• Level of acceptance and/or engagement with cultural and religious differences or pluralistic views (Barrelle, 2015).

Secondary outcomes
No secondary outcomes were included in this review.

| Duration of follow-up
Studies were included regardless of the length of follow-up after the intervention. If the length of follow-up varied across studies, we had planned to group and synthesise studies with similar follow-up durations. For example, short (e.g., 0-3 months postintervention), medium (>3 months, <6 months) and long-term follow-up (>6 months post intervention). However, no studies with follow-up data were located by the review.

| Types of settings
We aimed to include studies reporting on an impact evaluation of an eligible intervention using eligible participants, outcome(s) and an eligible research design in any setting. Where there were multiple conceptually distinct settings, we planned to synthesise the studies within the settings separately. However, there werer no variations in study settings.
We included studies written in any language that were identified by the search. We used Google Translate for the title and abstract screening stage to identify whether a non-English language study was potentially eligible for review. Due to resource limitations, potentially eligible studies that were published in a language other than English were not translated in full to enable screening them for final eligibility. However, we used Google Translate to translate as much of 3 We use the term "official data" loosely here as there is limited official data or direct measures of violent extremism. PIRUS stipulates strict inclusion criteria of individuals who have been radicalised or radicalised to violence. the document as possible to determine final eligibility. For transparency, we include the references to any language-other-than-English study that could not be unequivocally excluded using this approach in the "References to studies awaiting classification". We included studies published between 2002 and 2018 in the review.

| Searching other resources
We also employed additional strategies to extend the GPD search.
This included the following.
• Searching trial registries (those not indexed by WHO, but listed on the Office for Human Research Protections website https://www. hhs.gov/ohrp/international/clinical-trial-registries/index.html. • Searching counter-terrorism organisation websites (see Table 1).
• Conducting reference harvesting on existing reviews, eligible studies and studies deemed as closely eligible for the current review.
• Forward citation searching for all eligible documents.
• Liaising with the Five Country Research and Development Network (5RD), and the Department of Homeland Security Advisory Board network for the Campbell Collaboration grants, to enquire about eligible studies that may not be publicly available.
• Personally contacting prominent scholars in the field and authors of eligible studies to enquire about eligible studies not yet disseminated or published. 2. Record is not unique (i.e., duplicate).
3. Record is not about policing terrorism, radicalisation or extremism.
Prior to screening the entire corpus of search results, both screeners assessed the same small set of records and compared their judgements to verify consistent decision making. Although all efforts were made to remove ineligible document types and duplicates prior to screening, automated and manual cleaning can be less than perfect. As such, the first two exclusion criteria were used to remove ineligible document types and duplicates prior to screening each record on substantive content relevance.
Most records indexed in the GPD have a pre-existing full-text document. However, records from the additional searches that were deemed as potentially eligible at the title and abstract screening stage progressed to literature retrieval, where attempts were made to locate the full-text document. Where full-text documents could not be retrieved via existing university resources, they were ordered through the review authors' university libraries. All potentially eligible records then progressed to full-text eligibility screening. If the full-text document could not be located, the abstract was used to assess whether the study met full-text eligibility criteria. Where a decision could not unequivocally made about eligibility based on the abstract, the record was categorised as a study awaiting classification (see "References to studies awaiting classification" section).

Full-text eligibility screening
The same two review authors (EE and LH) also screened the full-text for potentially records for final eligibility according to the following exclusion criteria.
2. Document does not evaluate a policing intervention that aims to promote community connectedness.
3. The evaluation does not report violent extremism attitudes, beliefs or behaviour as an outcome.
All efforts were made to remove ineligible document types in earlier stages. However, sometimes these types of records can progress into later stages of screening (e.g., where duplicate records are not adjacent during screening or where screeners cannot unambiguously determine whether a record is ineligible based on the title and abstract). Therefore, the first two exclusion criteria were used to remove ineligible document types and duplicates.

| Data extraction and management
Two review authors (EE and LH) independently coded and extracted data from eligible studies within SysReview, using the coding form provided in Appendix D in the Supporting Information Material.
Disagreements or inconsistencies were resolved by discussion with a third review author (AH). Broadly, studies were coded according to the following domains.
1. General study characteristics (e.g., document type and study location).
6. Effect size data. 7. Risk of bias. (ROBINS-I), which guides rating across seven domains to determine low, moderate, serious, or critical risk of bias, or no information to make a judgement (Sterne et al., 2016). The confounding domain assesses whether the study accounts for the baseline and/or timevarying prognostic factors (e.g., socioeconomic status). The selection domain refers to biases internal to the study in terms of the exclusion of some participants, outcome events, for follow-up of some participants that is related to both intervention and outcome. The classification of interventions domain refers to differential (i.e., related to the outcome) or nondifferential (i.e., unrelated to the outcome) misclassification of the intervention status of participants. The measurement of outcomes domain assesses whether bias was introduced from differential (i.e., related to intervention status) or nondifferential (i.e., unrelated to intervention status) errors in the mea-

| Measures of treatment effect
There was only one eligible study, in which all outcome data were collected from individual participants, using continuous measures.
The specific data required to calculate effect sizes were not included in the eligible study reports, but was provided by the study authors.
Means, standard deviations and sample size for each group was used to calculate standardised mean differences (SMD) in RevMan, along with 95% confidence intervals.

| Dealing with missing data
Where data were missing in relation to coding categories, study authors were contacted by email to obtain the data. The results section also specifies which data were obtained from published reports of a study and which data were obtained directly from study authors (not available in the public domain).

| Assessment of heterogeneity
Due to the limited research included in this review, we were unable to assess heterogeneity. However, for updates of this review, our approach for handling these issues is specified in the review protocol.

| Assessment of reporting biases
Due to the limited research included in this review, we were unable to assess reporting/publication biases. However, for updates of this review, our approach for handling these issues is specified in the review protocol.

| Data synthesis
Due to the limited research included in this review, we were unable to conduct meta-analyses. Rather, estimates of treatment effects for eligible outcomes are presented as single SMDs, with their corresponding confidence intervals. For updates of this review, our approach for handling these issues is specified in the review protocol.

| Subgroup analysis and investigation of heterogeneity
We had planned to use subgroup analyses to assess whether the impact of the intervention varied by the following factors: geographical location, target population and type of policing strategy used to promote connectedness. However, due to the limited research included in this review, we were unable to conduct subgroup analyses. For updates of this review, our approach for conducting subgroup analyses is specified in the review protocol.

| Sensitivity analysis
We had planned to use sensitivity analyses to assess the impact of risk of bias on estimates of the treatment effect. However, due to the limited research included in this review, we were unable to conduct these analyses. For updates of this review, our approach for conducting sensitivity analyses is specified in the review protocol.

| Deviations from the protocol
Our review made six minor deviations from the protocol. First, de-spite intending to translate potentially eligible documents written in languages other than English, resources did not permit this approach.
However, for transparency, the "References to studies awaiting classification" section lists all studies written in another language that could not be unequivocally excluded on their titles and/or abstracts or by attempts to translate documents via Google Translate.
Our second deviation was changing the hand searches of journals to 12 months prior to the end search date within the GPD and grey literature sources (i.e., December 2018). The protocol stated that the hand search would encompass the most recent four issues of each of the listed journals; however, conducting the hand search in this way would have included a nonequivalent sample of research (i.e., 2019 data, which was not captured by the remainder of the systematic search).
Our third deviation was using the full-text eligibility criteria to screen the titles and/or abstracts of records that were retained after title and abstract screening, but where a full-text could not be sourced to determine final eligibility. Records were only excluded in this way if an unequivocal decision could be made, otherwise, they were retained and included in the "References to studies awaiting classification" section.
Our fourth deviation was not contacting study authors where there was either missing information or "unclear" ratings during the risk of bias assessment. We chose this approach as the additional information would not have changed the overall risk of bias result. However, for updates of the review, we will follow the original protocol.
The fifth deviation was slightly changing the wording of the title and abstract screening criteria. In the protocol, the third criterion was "Document is not about terrorism or extremism". While this wording was appropriate for records extracted from the GPD search because all records were impact evaluations relating to police or policing, this approach was overly sensitive for results captured by the grey literature and hand searching. Therefore, we changed the third title and abstract screening criterion to "Document is not about policing terrorism, radicalisation or extremism".
The final deviation to the protocol is adding additional search steps in light of the low number of eligible studies identified by the systematic search and screening process. Specifically, we harvested the reference lists of excluded studies that were deemed to almost meet inclusion criteria (e.g., eligible intervention, no evaluation or ineligible outcomes) or that contained substantive content that was closely aligned with the review topic.

| Included studies
The sole eligible study by Williams, Horgan, and Evans (2016)  The authors report that the groups were matched using propensity score matching using demographic characteristics age, race, religion and educational level; however, it is unclear whether this was  Williams et al. (2016) assessed the WORDE intervention on behavioural outcomes (e.g., increased coping skills), attitudes (e.g., towards different religions or ethnicities) and knowledge of outgroup cultures. Self-report surveys captured these outcomes at one time point (after an unknown period of engagement with the programme 4 ) using a 14-item measure constructed by the authors, the Brief Volunteer Program Outcome Assessment. Items were measured on a seven-point Likert scale ("completely disagree" to "completely agree") with higher mean values equating to more agreement with the statement. The specific items were as follows.
"Thinking of when you volunteer, please rate your level of agreement with the following statements: • I feel welcome* • I feel a part of something bigger than myself* • I feel a sense of teamwork* • I make friendships that are active beyond the event* • I make friends with people from other races* • I feel useful • I have responsibilities • I have leadership responsibilities • I feel a sense of purpose • I feel free of peer pressure* • I feel accepted*

• I wouldn't feel lonely
• I wouldn't feel afraid to talk to others* • I learn about cultures other than my own*" (p. 157) Of the above items, nine were deemed as eligible outcomes (marked with asterisk) and were considered to fall under the general banner of deradicalization. Specifically, these items align with Barrelle's (2015) conceptualisation that is defined as the level of acceptance and/or engagement with cultural and religious differences or pluralistic views and modification of group or personal identity. It is important to note that it is not explicitly clear whether the evaluation participants in the treatment group were all directly exposed to the two intervention components that involved police. Hence, these evaluation outcomes may not be direct measures of how effective police were at countering violent extremism by promoting community connectiveness.

| Excluded studies
Due to the number of records screened for final eligibility (n = 1,057), we are unable to describe the full body of excluded studies (see "References to excluded studies" section). Rather, this section describes a small set of studies that almost met the review inclusion criteria or described interventions closely aligned with the review topic (n = 29). A total of 24 were documents that contained substantive content about community-oriented policing approaches for countering terrorism, extremism or radicalisation but that did not report on an original impact evaluation ( Example intervention approaches included the following. • Multiple community policing activities in Amhara, Ethiopia, focus on trust-building, including organising watch patrols, adjudicating minor disputes, literacy training, public health and family education (Schwartz, 2015).
• Hotlines or online reporting for community to notify police of concerns relating to possible terrorist activity (Briggs, 2010).
• Engaging the community in dialogue around countering violent extremism via social media ).
In addition, two studies reported on impact evaluations of eligible interventions but used ineligible outcomes to assess the 4 Information provided via personal correspondence with study authors.

MAZEROLLE ET AL.
| 13 of 48 intervention's effectiveness. For example, Aksu (2014) evaluated the impact of nine community policing activities (e.g., family visits, sporting events, visits to police units) within a counter-terrorism context in Turkey using an unmatched control group on citizen perceptions of police procedural justice and legitimacy (see also .
A sample of three studies also examined eligible intervention approaches but did not evaluate these using eligible research designs  Table 2 summarises the degree of bias and specific reasons for this rating across the seven domains, based on the standardised questions provided by the ROBINS-I tool. Specifically, the study was rated as having serious risk of bias for the confounding, selection, classification of interventions, and measurement of outcomes domains. A lack of information or ambiguity in the available reports for the study or data provided by study authors led to a rating of "no information"

| Risk of bias in included studies
for the deviations from intended interventions and the missing data domains. Finally, the selection of reported results was rated as having a moderate risk of bias. Overall, these ratings suggest that the single included study included in the review has a relatively serious risk of bias. 6 | DISCUSSION

| Summary of main results
Police programmes to tackle violent extremism can involve a range of approaches and partnerships. One approach includes efforts to improve community connectedness by working to address social isolation, belonging, economic opportunities, and norms and values that may lead people to endorse or support violent extremist causes and groups. The assumption is that the risk of an individual being radicalised in the community can be reduced when police work with community members and groups to mobilise and support activities that help generate a sense of belonging and trust. Police programmes that build a sense of belonging and trust may help ensure individuals are not influenced by activities that violent extremists use to attract support for their cause.
Our review identified 24 studies that nearly met the inclusion criteria but were ultimately excluded because they did not use rigorous experimental or quasi-experimental evaluation methods. These studies are summarised in this report and indicate a wide array of practices that are being adopted by police in their efforts to improve community connectedness as a way to tackle violent extremism.
Our review only identified one study that met our inclusion criteria. The included study was conducted in the United States by a Muslim-led organisation in partnership with police and other agencies. The programme aimed to counter violent extremism through an education and awareness campaign and by improving networks for agencies/third parties to help assist individuals identified as at-risk of radicalisation. The awareness raising activities aimed to increase knowledge and understanding about violent extremism risk factors that would help to facilitate problem solving between police and community groups to better to identify risks within the community.
Increasing prosocial linkages or ties between agencies, community members and police was aimed at ensuring these stakeholders could better work together when identifying and coming into contact with radicalised individuals. The purpose was to help generate greater involvement in responses to terrorism across various stakeholders.
The included study used a quasi-experimental design to compare programme participants to non-participants on a range of self-report measures comprised of items representative of deradicalization. Of the nine effect sizes calculated, all favoured the treatment group except for one which favoured the comparison group (survey item: "I make friends with people from other races"). The effect sizes were small to medium in size and only three did not have confidence intervals that included zero. It is important to note that it is not explicitly clear whether the evaluation participants in the treatment group were all directly exposed to the two intervention components that involved police. Hence, these evaluation outcomes may not be direct measures of how effective police were at countering violent extremism by promoting community connectiveness. Overall, our review demonstrates that there is a global lack of rigorous evaluation evidence to conclusively determine whether or not police-involved programmes to tackle violent extremism through promoting community connectedness are effective. programme), which raises the risk of time-varying confounding because participants may have received both interventions. While the authors state that propensity score matching was used, their published report and data provided by the authors for the purposes of this review do not allow for independent assessment of the propensity score matching. Specifically, the variables the authors use to "match" participants vary within the report are not clear. For example, one section states participants were matched on demographics and another section states the participants were matched on nine measures including: trust in police, political extremism, racism, resiliency and coping, emotional stability, amped political extremism, historical loss, religiosity, and religious dogmatism. The time of measurement for these potential confounders is also not clear (e.g., whether at the beginning and end of the intervention).

Selection Serious
Participants were invited to participate in the evaluation after they had already begun engagement with the WORDE programme (treatment) and the treatment group was based on a stratified random sample from those who expressed interest in the study. One of the variables influencing selection included the frequency of attendance. Although authors sampled across regular, infrequent and "one-timer" attendees, it is unclear how attendance might impact estimates of treatment effect.

Classification of interventions Serious
Intervention and comparison group not clearly defined. Specifically, it is unclear whether authors verified whether comparison group participants had any contact or engagement with the WORDE programme. The specific treatment received by comparison group participants is not clearly articulated aside from other multicultural events or volunteerism activities. It is also unclear whether the information used to define the groups was recorded at the start of the intervention or after the intervention and evaluation was underway.

No information
The intended intervention and actual implementation of intervention were reported by study authors. The authors outlined the aims and activities of WORDE programme, but do not discuss implementation challenges or changes made to implementation. The reported variation in attendance for WORDE participants is suggestive of adherence issues, but this is not unexpected in community interventions and the content of the intervention (countering violent extremism).

Missing data No information
The published report of the study does not provide participant demographics. Data supplied by study authors suggests there was no attrition, but this could be not be independently verified.

Measurement of outcomes Serious
Outcome measurement appears to be equivalent across groups. However, study authors developed the outcome measures based on interviews with WORDE participants (treatment group) and it is not clear whether the same participants are used for the evaluation and design of the instrument. In addition, the way participants answered the survey questions may have been influenced by their knowledge of and self-selection into the treatment and evaluation.

Selection of reported result Moderate
No prospectively published protocol for the study exists. The published report for the study does not provide the results of the between group analyses, but rather reports data for the treatment group and a statement that there were no statistically significant differences between the treatment and comparison group. Given this statement about lack of differences, it is unlikely that the reported effects or data provided by study authors for this review are selected on the basis of the results from multiple outcome measurements within the one domain, multiple analyses of the intervention-outcome relationship, or different subgroups. Data was provided by authors upon request. location. It was not possible to assess the differential impact of policing interventions that target either individuals or places (either micro or macro).
The outcome measures included in the one eligible study were limited to self-report attitudinal measures, which also limits our capacity to examine the full breadth of outcomes, including: direct behavioural outcomes representative of violent extremism, different components theorised to fall within radicalisation and disengagement, and outcomes measures in different modalities such as official administrative data. In addition, self-reporting measures may increase the potential for bias specifically in relation to radical attitudes, behaviours or beliefs. Due to the high-risk nature of this behaviour participants are less likely to answer on a truthful basis, especially if they do espouse radical beliefs and attitudes that could escalate to violence. Finally, the single included study was a multicomponent intervention whereby not all intervention components explicitly had police involvement.
Therefore, any effects of the intervention on the measured outcomes may not be directly attributable to police efforts to enhance community connectedness.

| Quality of the evidence
The single included study in this review was rated as having serious risk of bias, using the ROBINS-I tool. The overall quality of evidence on the effects of police programmes to tackle violent extremism through efforts to improve community connectedness is weak and T A B L E 3 Impact of World Organisation for Resource Development Education programme on self-report deradicalization outcomes Therefore, it will be important that this review is updated within 2-3 years to capture any new research, which will also facilitate more concrete conclusions about the effectiveness of policing interventions aimed at enhancing community connectedness to counter violent extremism.
6.5 | Agreements and disagreements with other studies or reviews

| Implications for practice and policy
Despite the findings of this review, further research is needed to ascertain whether police efforts to enhance community connectedness can counter violent extremism attitudes and beliefs. With the lack of an existing evidence base, a key question to be raised is the role of the police in such community-based efforts. For instance, existing scholarship draws attention to the downside of counterterrorism community engagement with the Muslim community, which has seen police prioritise intelligence gathering on terrorist threats over more collaborative efforts that aim to address issues of concern to Muslim community members (Cherney & Hartley, 2017).
Moreover, since 9/11, the war on terror has sometimes led to a deterioration in relations between the police and the Muslim community due to perceptions of police unfairly targeting Muslims . This erodes trust towards and cooperation with the police. This does not mean police should play no role in strategies to enhance community connectedness to tackle violent extremism. Yet it does raise the issue as to what precisely should be the role of police in managing complex issues such as violent extremism within their own communities (see Meares & Tyler, 2020).
Police often come into contact with individuals only after they have engaged in radical actions or behaviours. This suggests that police could be well placed to cultivate communities of "intimates" around those that are exhibiting radical characteristics (see Grossman, Stephenson, Street, & Zhang, 2015). Family members, close friends and community insiders are best placed to detect signs of change in thinking and behaviour and as Grossman and her colleagues (Grossman et al., 2016;Thomas, Grossman, Miah, & Christmann, 2017) suggest, there is much to be gained by police doing much more than they presently do to en-

| Implications for research
Conducting evaluation research in the area of countering violent extremism is particularly difficult (Koehler, 2017;Romaniuk & Chowdhury Fink, 2012   Disley, E., Pardal, M., Weed, K., & Reding, A. (2016). Using multi agency public protection arrangements to manage and supervise terrorist offenders: Findings from an exploratory study. Santa Monica, CA: RAND Corporation.