Scottish prison population projections: September 2024

2 Background

The ‘microsimulation’ scenario modelling approach is to simulate arrivals and departures of individuals to and from the prison population to help estimate 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’ custody status over time.

Data and assumptions

To produce outputs, the model draws on selected court activity data, for example, levels of court hearings at different stages (such as pre-trial or 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 one of the most important factors for prison population changes in the short/medium term.

Modelling limitations

As the model can be validated against historical data and can reflect a wide variety of dynamics, it is suitable for forecasting 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 reduced by the recovery programme) which impacts the model’s assumptions and longer-term predictive power, currently only a 6-month projection is generated using the model. The model does not currently simulate flows for different crime-types, so crime-based trends are not explicitly modelled.

Report structure

The rest of this report is organised as follows. Section 3 includes a brief overview of the prison population from 2020 to 2024 and court demand trends from 2014 to 2024. Section 4 includes an overview of the assumptions used to project the prison population and explains how the model’s previous projections are validated for robustness against the actual prison population in recent months. Section 5 summarises the population projection results for the period from August 2024 to January 2025. Section 6 features a brief discussion of factors which may influence the size of the prison population in the longer term. Finally, the report concludes with a short annex covering some technical aspects of the modelling approach and how the scenario variant assumptions are derived.