Tool uses the information of a prisoner’s age and previous behaviour to project how many incidents they will become involved in, supporting categorisation decisions and direction of care and support
HM Prison and Probation Service has shed light on the operation of an algorithm used to estimate the likelihood of individual inmates to engage in violent behaviour.
Details of the function of the Violence in Prisons Estimator have been released in a transparency document published via GOV.UK’s repository of public-sector algorithmic records. The HMPPS tool “produces an estimate of the number of violent incidents that an offender in custody is at risk of being involved in over the next year, based on their age and previous behaviour in custody”, the record says.
The factors that define the outcomes produced by the system were chosen “due to substantial evidence linking them to incidents of violence in custody”, the document adds.
The projections made by the automated platform are then taken into account by frontline prison workers – about 2,000 of which access the algorithm each month – and, “in conjunction with other data sources, [used] to quickly identify whether an offender is likely to be involved in violence whilst in custody and how frequently, [which] enables HMPPS staff to prioritise their resources effectively and improve the safety within prisons”.
For each offender, the tool issues an estimate in the form of a round number – “for example, if someone has an estimate of 2, this means that, historically, offenders with a similar profile – in terms of age and history of incidents – were involved in an average of 2 incidents in the following year”.
This information is then used by prison workers to assist them to “assess the violence risk of new arrivals into custody” and “to help identify which offenders might benefit from further support to manage their risk of violence”. It also enables “staff to prioritise further analysis”, the record explains.
The estimates made by the system are also incorporated in the Prison Categorisation tool which is designed to help staff “make decisions about what prison category an individual should have… for example, the tool may recommend that an individual who has perpetrated a large number of assaults whilst in custody and has a high violence in prisons estimate, should be placed in a higher category prison than an individual who does not have these characteristics”.
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In the 12-month period up to the end of November 2024, the wider categorisation tool was used to support 50,000 initial categorisation decisions, and a further 83,000 recategorisation decisions.
The transparency record stipulates that “a human decision-maker is involved in every process where these estimates are used”. It adds that “no punitive decisions or automated decisions are taken solely on the basis of the tool’s estimates”.
“The tool is best understood as a more stable and informative version of an assault rate — it offers a high-level view of risk based on past behaviour and age, but it must be used with caution and in context, not as a definitive measure,” the document says.
Support and wellbeing sessions
The prison service first began experimenting with the use of an algorithm to estimate violence in 2019.
The automated system has replaced a process in which, previously, “staff had to spend a significant amount of time manually compiling data to identify the offenders with the greatest risk of violence in custody”.
The algorithmic record adds: “There was often not time to produce this analysis for every offender, which meant that some individuals with a greater risk of violence may not have been initially identified, reducing the opportunity for staff to take proactive, preventative action. There was also no uniform way to compare the assessments of violence risk produced by different teams or prisons.”
The document cites a range of benefits that have been delivered by the implementation of the tool, including the enablement of “a data-driven violence management process [which] provides prison staff with a consistent and robust, data-driven way to compare violence levels and risk of their population”.
This, in turn, helps staff better target support for certain prisoners – while also eliminating the need for “hours of manual work”.
“Prison staff have said that the tool’s estimates help them to be proactive and target the right groups of prisoners for support and wellbeing sessions, and that they see it as an important part of their toolkit for managing and reducing violence in custody,” the record says.
In response to enquiries from PublicTechnology, a spokesperson for HMPSS parent department the Ministry of Justice said: “As the public would rightly expect, we regularly deploy technology to better protect our staff and keep the public safe.”
