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Why is it taking the police longer to investigate crime? Crest's approach to quantifying complexity

Juliet Netting, Strategy and Policy Projects Lead | Ellie Covell, Strategy and Insight Manager

Friday 25 October 2019

Is it possible to quantify the impact of complexity on police investigations? That is one of the questions Crest is seeking to explore as we continue to expand our demand modelling: specifically, to what extent vulnerability and evidential factors complicate cases and increase the amount of time police spend on them, and how understanding the likelihood of these factors can help forces better plan for future demands.

We noted in our last blog that police are seeing a rise in complex crimes involving vulnerable people, such as sexual abuse and domestic abuse, which take longer to investigate, and therefore increase demand pressures on forces. We predict that these offences will continue to increase, meaning forces will have to make changes to the composition of the workforce. In fact, our work with Nottinghamshire and Bedfordshire Police suggests that the growth in complexity is not limited to ‘high harm’ offences - but is affecting all types of offences, meaning the police are spending longer dealing with all types of crime.

“What may have been a straightforward investigation 10 years ago now has a raft of new investigative opportunities”.

Cressida Dick, Commissioner of the Metropolitan Police(1)

As part of our demand modelling work with both forces, we conducted focus groups across the command structure and developed a survey for officers to respond to.

We asked officers whether the secondary investigation of an offence was likely to involve time demands not directly linked to the investigation. Predictably, respondents reported that cases such as rape and sexual offences were very likely to require further time demands. Interestingly though, other cases were also considered likely to require further work; more than 50% of officers surveyed in Nottinghamshire agreed that burglary and robbery cases would require work not directly linked to the investigation.

Proportion of survey respondents (n=126) in Nottinghamshire agreeing that the secondary investigation of an offence was likely to involve work not directly linked to the investigation (i.e. rehousing)

The extra work that a case may require is due to what we term ‘complexity factors’. These are aspects of the case, including the characteristics of the victim, offender, or type of offence, which may require the officer to spend additional time on the case. For example, a burglary case may involve an underage or elderly victim whose vulnerable status means that the police may need to take time contacting relatives and carrying out safeguarding arrangements which would less likely be necessary in a case involving a healthy, middle aged adult. All of this means that the type of offence isn’t necessarily a good indicator of complexity and/ or how long it will take the police to resolve the case.

“it is impossible to say by offence type - it can be a straight forward domestic complaint may be easily dealt with and then a minor theft can end up taking hours to sort”

Police Sergeant, Prisoner Handling Team, Nottinghamshire

Complexity Factors


Our work with Nottinghamshire suggests to us that there are two aspects to complexity which affect police demand: vulnerability and evidence.

These broad categories can be broken down further into the specific issues listed below.

Vulnerability factors:

  • Characteristics of individuals involved

  • Mental Health

  • Drugs/Alcohol

Evidence complexity:

  • Digital evidence availability

  • No clearly defined locations/times of offence

  • CPS requests for detailed offending history/contextual information

While there are various other factors that can complicate a case, such as an individual’s past experiences with crime and victim/suspect cooperation, as a starting point we explored why these factors can impact the amount of time a case requires.

Vulnerability factors

"Any offence involving violence, domestic violence, or violence to children, has a number of safeguarding issues to deal with, which are not directly related to the investigation. Often, safeguarding issues takes up a disproportionate amount of time when compared with the offence itself"

Detective Constable, CID, Nottinghamshire

"The levels of safeguarding and multi-agency cooperation mean that virtually every crime type will have some 'hidden' element of work to it that the Control Room may not be aware of. This may go on for some days depending on the nature of the incident and be juggled along with other demand."

Police Sergeant, Neighbourhood Policing Team, Nottinghamshire

Characteristics of individuals involved


The age of any individual involved in a case - the victim, witness or suspect - can all complicate an investigation. Both the young and the elderly have specific needs which can increase the amount of police time required to hand a case.

Young people involved in a police investigation clearly require further actions to safeguard, whether they are a victim or a suspect. A guardian may need to be contacted, or other local organisations may need to be involved in the case to ensure the young person is cared for.

A recent investigation into crimes against older people identified specific challenges that older victims may present to investigators.(2) These challenges often involve supporting the victim to communicate evidence, with needs ranging from digital assistance to participate in a video interview to the requirement for a registered intermediary to help the victim understand the investigatory process and communicate with the police and court. The report by HMCPSI and HMICFRS notes that these complexities are significant enough to require officers to have special training, and that specialist protecting vulnerable people units could be more effective at conducting effective investigations for these victims.

Because some crimes may be more prevalent in certain age groups than others (for example, older people are more likely to fall victim to fraud(3)), it is theoretically possible to factor this into demand assumptions for that type of crime.

In a similar vein, the number of victims/witnesses/suspects involved in an incident will also clearly affect the workload associated with the incident. Whilst for some offences you can expect a large number of people to be involved (some public order offences), it is less easy to predict with others. For example, aggravated assault may happen in an isolated location and have one victim and one suspect, or it could occur in a busy street full of witnesses, and have many possible suspects. The second case would take much longer to investigate than the first, despite being examples of the same offence. This is, of course, obvious to the officers who handle these situations on a daily basis. Our modelling will enable Chief Constables to quantify this demand and the implications for staff allocations.

Mental health of individuals


A case can also be complicated by any other vulnerabilities of an individual involved in an investigation. As with age, these factors may be most prevalent in certain types of cases - such as mental health call outs - but can affect any case. In July 2016-June 2017, 2.4% of all recorded crime in England and Wales was flagged to identify individuals with mental health concerns.(4) The increasing demand of managing vulnerability has been recognised by the Home Affairs Committee:

“the police service is playing an increasing role in managing vulnerability and risk across public services, and many individuals have complex needs which cross organisational boundaries.”(5)

The increasing demands on police resources from mental health issues in particular has drawn attention in recent years. HMICFRS’s report found that nearly all police forces saw in increase in mental health demand, but notes that this increase is also influenced by increased public and staff awareness of mental health conditions. The report found that, on average, when a case is marked with a mental health flag more officers are sent to in and it takes longer to resolve. Research has also shown that people with severe mental health issues are more likely to be victims of crime than the general population.(6)

Alcohol and drugs


Public Health England reported in March 2019 that the latest prevalence estimates of opiate and crack use (OCU) showed that at a national level, the combined numbers of people who take crack cocaine on its own, illicit opiates (mainly heroin) on their own and those who take both drugs, rose by 4.4% between 2014-15 and 2016-17. National Drug Treatment Monitoring System (NDTMS) also shows that the numbers of people coming into treatment for non-opiate problems increased between 2014-15 and 2017-18.

Numbers of people entering substance misuse treatment, 2009/10-2017/18

At the same time the proportion of people leaving substance misuse treatment free of dependency has been declining.

Proportion of people leaving substance misuse treatment free of dependency, 2009/10-2017/18(8)

Dealing with a victim or perpetrator under the influence of drugs and alcohol is an issue which crosses between vulnerability and evidence complexity. Having either party under the influence will of course affect the standard of the evidence available and the ability to capture it. Outside of the investigation itself, there will be necessary safeguarding measures required such as ensuring the individual's immediate safety by accompanying them to hospital or referring them for longer term support:

"Qualifiers such as alcohol, drugs and mental health can alter how long a matter takes to be dealt with dependant on them at the time of arrest. There will also be meetings with partnership agencies and referrals to address any drug/alcohol/mental health."

Police Constable, Neighbourhood Policing Team, Nottinghamshire

Understanding the demographics of an area and how they correspond to types of offence may be one way to take vulnerability complexities into account when anticipating demand. For example, if you know that an area has a high proportion of individuals with alcohol dependency, and that these individuals are most often associated with a specific case type, then you can anticipate that those cases may take more police time, or that the local police should receive training to help them effectively support those individuals. The system of flagging particular characteristics in a case, such as a victim/suspect having mental health issues, sets the groundwork for improving demand forecasting, and can be built upon to help police forces understand what resources they need in future.

Evidence Complexity


There are many factors which can directly affect the time required for the police to complete an investigation. These factors are more difficult to attribute to particular types of offences, as they can affect any case.

“A 'simple' Robbery can become complex due to availability of witnesses, amount of digital evidence and location of the crime which requires extensive CCTV work, this can be made more complex with the needs of the victim and the suspect. Demand can affect the way in which we investigate crime, as a simple job can be made to feel complex because of demands from other crimes that pull you away from what you are currently dealing with.”

Detective Constable, CID, Nottinghamshire

Digital evidence availability


The volume and availability of digital evidence related to a case can vastly increase the amount of police time required for investigation. Downloading the data from a smartphone can take five to seven hours, and then days or weeks of combing through the evidence it contains. The number of digital devices involved in a case can vary widely, but can be involved in some way in almost any case.

Crest’s work with Nottinghamshire Police found that initial demand modelling underestimated the amount of time required for investigating digital evidence across several types of offence. Respondents to our survey thought that drugs trafficking and possession were the most likely offence types to be affected by digital evidence complexity, but commented that digital evidence can impact any crime.

Offence types which respondents (n=129) in Nottinghamshire agreed were most likely to be affected by digital evidence complexity

No clearly defined locations/times of offence


Simple things can also be regarded as complexity factors in a case, such as when police lack information about exactly when or where an offence took place. For example, if a bike is stolen but there is a 12-hour time frame in which it could have been taken, the police must spend much more time checking CCTV footage than would have been necessary if the time frame was shorter.

Police forces have reportedly been attempting to minimise the amount of time they spend on low level offences which would require a disproportionate amount of time to investigate. However, a lack of clearly defined location and time for an offence remains a complicating factor in all case types.

CPS requests for detailed offending history/contextual information


Other ways that cases can require more police officer time are requests from other criminal justice agencies. For instance, the Crown Prosecution Service may ask the force for a full criminal history of an offender, requiring more police time to provide.

The complexity of the evidence can vary greatly from case to case, and so further investigation is required to understand how these factors can be considered in the context of predicting demand. Understanding these complexity factors could help forces better utilise their resources and plan for the future.



Our work so far has found solid evidence that police work has become more and more complex across all types of demand (both dealing with incidents in the immediate term and secondary investigations). Adding to this years of reduced funding and increasing crime rates, forces are now facing enormous challenges in meeting demand. Levels of complexity are showing little sign of slowing down or reversing, with our increasing use of digital communications, rising drug use and mental health issues.

It is imperative that police are equipped to support vulnerable individuals in the criminal justice system AND provided with the enablers required to effectively investigate complex crime. Central government should support forces to ensure the additional allocations of officers are used to best effect and adequately trained to deal with these complexity factors. This should also be used to open a conversation with the public - the pressures of rising complexity on the police has rarely been described or explained to the public in any detail and this must be addressed urgently.



1. John Harris Memorial Lecture, for the Policing Foundation

2. HMCPSI and HMICFRS, (2019), The Poor relation; The police and CPS response to crimes against older people,

3. ibid

4. HMICFRS, (2018), Policing and Mental Health; Picking up the Pieces,

5. House of Commons Home Affairs Committee, (2018), Policing for the future,

6. Victim Support & Mind, (2013), At risk, yet dismissed; The criminal victimisation of people with mental health problems,


8. ibid

9. Helen Warrell for the Financial Times, (Aug. 2019), How Digital Forensics is Transforming the Metropolitan Police,

10. Helen Warrell for the Financial Times, (Aug. 2019), How Digital Forensics is Transforming the Metropolitan Police,


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