Prison population projections: November 2023 edition

2 Background

The ‘microsimulation’ scenario modelling approach adopted for producing the projections, simulates arrivals and departures of individuals to and from the prison population and then estimates the number of individuals in prison on a particular date in the future. The microsimulation model uses individual-level data to construct a representation of the population of interest. The model can simulate, using known probabilities, changes in individuals’ status over time¹.

To produce outputs, the model draws on particular court activity data, for example, levels of court hearings at different stages (pre-trial/trial) in High Court, Sheriff Court Summary and Sheriff Court Solemn. This data is used to estimate levels of remand arrivals in the coming months. The model includes assumptions on future court capacity, as these are currently the most important factor that is likely to impact on prison population changes in the short/medium term. The model does not currently simulate flows for different crime-types, so crime-based trends are not explicitly modelled. As the model can be validated against historical data and can reflect a wide variety of dynamics, it is suitable for forecasting both in the short and medium term. However, due to uncertainty underlying the justice system’s recovery (e.g., the rate at which court backlogs can be tackled) which impacts the model’s assumptions and longer-term predictive power, currently only a 6-month projection is provided.

The rest of this report is organised as follows. Section 3 provides a brief overview of the prison population from 2020 to 2023 and court demand trends from 2014-15 to 2022-23. Section 4 provides an overview of the assumptions used to project the prison population and explains how the model’s previous projections are validated for robustness against actual prison population figures in recent months. Section 5 summarises the population projection results for the period from October 2023 to March 2024. Section 6 concludes the report and briefly discusses factors which may influence the size of the prison population in the longer term.