The Offender Assessment System combines a structured manual assessment, completed by a prison or probation officer, with actuarial calculators. The manual part covers offence analysis, accommodation, education and employment, finances, relationships, lifestyle, drugs, alcohol, emotional wellbeing, thinking and behaviour, attitudes and health. The statistical part includes OGRS 3, which takes gender, number of previous sanctions, age at current conviction, age at first sanction and offence type, and outputs a percentage likelihood of any proven reoffending at one and two years; and OGP and OVP, which combine the OGRS 3 score with questionnaire sub-scores to predict general and violent reoffending.

The algorithms were built from probation and prison caseload systems together with the Police National Computer. Outputs classify offending-related needs and risk, and feed directly into sentence planning, prison categorisation and supervision decisions.

It is used on more than 1,300 people per day in England and Wales, has been in use since 2001 and was rolled out across the prison and probation system between 2001 and 2005. That scale and longevity make it arguably the most consequential algorithmic system in the UK criminal justice process, and among the least publicly discussed. A replacement, Assess Risks Needs and Strengths, is being built in-house by Ministry of Justice Digital.

The Ministry of Justice has published revalidation studies of the actuarial instruments. Concerns about accuracy and discrimination have been raised by Statewatch and in trade reporting.