4. Introduction to Prison Population Projections

This section provides an overview of the assumptions and scenarios developed 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. The first issue of the Scottish prison population projections[1] provides a more detailed overview of the modelling technique used.

Model Assumptions and Scenarios

The projection scenarios are based on a variety of assumptions about how the rate of transitions into and out of the prison population might change. The assumptions depend on trends and planned changes to the court system, including court capacity changes, increased court conclusions rate, increased remand arrivals, remand/bail mix and prioritisation of remand case progression through courts. Departures from the sentenced population are estimated using a combination of court disposal data and snapshots of the prison population.

To help with understanding how sensitive the size of the prison population may be to variation in court case conclusion rate, three variants have again been included in the modelling - central, higher, and lower throughput. The “central” court throughput scenario variant assumes case conclusion rates per courtroom which are based on data covering the twelve-month period from March 2024 to February 2025. The “higher” scenario assumes the average case throughput per court will be slightly greater than it has been over the same period, and the “lower” scenario assumes that the average case throughput per court will be slightly smaller. The three scenario variants are shown in Table 1. The pandemic led to disruption of the justice system, for example criminal court business was temporarily restricted to essential business only[2]. When combined with an increase in court business, this has led to a backlog of trials yet to be held. Court recovery resources have helped reduce the backlog from the peak level, but it nevertheless is still high compared to before the pandemic. This means that the justice system is not in balance, which makes it more challenging to estimate future rates of case conclusions and imprisonment disposals. Therefore, since the progression of the justice system’s recovery (e.g., rate of reduction of scheduled trials) impacts the model’s assumptions and longer-term predictive power, the projections cover a limited period, from April 2025 to September 2025. Remand arrivals are sampled from March 2024 to February 2025.

Table 1. Prison population scenario variants.

Model Quality Assurance

At each update, the previous projections are compared with the actual population to determine the model’s suitability to continue providing reliable projections. Figure 14 shows the November 2024 projections from the microsimulation, which were published in December 2024[3]. Even after accounting for a wide variety of uncertain dynamics (including seasonal effects around December/January) in the system, Figure 14 indicates that the projected ranges for the remand, sentenced and total populations reasonably accurately aligned with the actual levels up until 18 February 2025, when the first tranche of Early Release took place[4] (the projections published in December did not account for the expected impact of the short term prisoner early release act 2025). The actual remand population remained near the centre of the range from November until the beginning of March 2025.

Figure 14. Prison population projections and actual prison population beginning on 1st November 2024, based on courts and prison population data up to the end of October 2024. The actual population is shown as a series of black points up to 1st April 2025.

To check the model further, back-casting is used to retrospectively compare the actual prison population for the past few months against a projection generated by the model based on actual monthly court throughput data. The back-cast eliminates uncertainty about the majority of the assumptions, so if there was a difference between the back-cast and the actuals it may indicate technical deficiencies in the model. The recent projection presented in Figure 15 shows that the back-cast from August 2024 to January 2025 is largely accurate.

Figure 15. The back-cast is based on remand arrival and courts throughput data from August 2024 to January 2025. The actual population is shown as a series of black points.

Microsimulation Model Limitations

The model relies on the availability of a large amount of frequently refreshed high-quality data about court activity and prison populations, some of which can be resource intensive to obtain and process.

The model does not currently simulate flows for different crime-types, so crime-based trends are not explicitly modelled. However, there are plans to develop the model further and include case-mix in future modelling.