Scottish prison population projections: December 2024

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.

Modelling 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.

For the first issue (June 2023) of the Scottish prison population projections[2] only three scenarios were developed, using central remand arrivals and either low, central or high court throughput. Subsequent editions included a further six scenarios combining the original three court throughput scenarios with higher and lower remand arrival scenario variants. At the start of 2024 remand arrivals were elevated , especially from April to July 2024, and this appears to be a consistent pattern across the years with elevated remand arrivals near the start of the year. Therefore, in this edition of the projections it is assumed that remand arrivals will be similar to February 2024 to July 2024 for the part projected period, from February 2025 to April 2025. A review of previous trends also shows that a general seasonal pattern tends to occur at the end of the year which is associated with irregular court throughput and remand arrivals during the months of December and January[3]. To account for this, in the latest projections the assumptions for December 2024 are sampled from December 2023 and the assumptions for January 2025 are from January 2024.

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 November 2023 to October 2024. 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[4]. Alongside 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 by almost 50% 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 November 2024 to April 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 13 shows the August 2024 projections from the microsimulation, which were published in October 2024[5]. Even after accounting for a wide variety of uncertain dynamics in the system, Figure 13 indicates that the projected ranges for the remand, sentenced and total populations reasonably accurately aligned with the actual levels up until November 1st. The actual remand population has decreased, but remained near the centre of the range from August to the beginning of November 2024.

Solemn court procedure throughput was high in recent months (see Figure 7) and has contributed to the growth seen in the sentenced population in every month of 2024 (except when ER was underway). As shown in Figure 13, the total and sentenced populations increased sharply following ER, but then stabilised somewhat such that the total population was near the middle of the range at the beginning of November.

Figure 13. Prison population projections and actual prison population beginning on 1st August 2024, based on courts and prison population data up to end of July 2024. The actual population is shown as a series of black points up to 1st November.

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.

During June and July 2024 ER[6] was implemented for eligible individuals on short-term sentences who were already due for release in the near future. This action was taken to ease pressure on the prison estate, following substantial population growth during 2023 and early 2024. ER[7] led directly to 477 individuals departing during June/July 2024. The implementation of ER meant that July 2024 was atypical for flows into and out of the population. Hence the back cast omits data from July 2024 and uses data sampled from July 2023 instead.

The recent back-cast projection presented in Figure 14 shows that the back-cast from April 2024 to September 2024 is largely accurate (although the actual sentenced population at the end of September is slightly lower than the projected range, primarily due to slightly underestimating the impact of ER).

Figure 14. The back-cast based on remand arrival and courts throughput data from April to September 2024. 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.