Scottish prison population projections: October 2025

This section provides an overview of the assumptions and scenarios used to project the prison population, and explains how the model’s previous projections are validated against actual prison population figures to assess robustness. 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 range of assumptions about how the rate of transitions into and out of the prison population might change. The assumptions reflect trends and planned changes in the court system, including: changes in court capacity; variations in court conclusions rate; fluctuations in remand arrivals; the remand/bail mix; and, the 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 assess the sensitivity of the prison population to variation in court case conclusion rates, three scenario variants are included in the modelling:

• “Central” throughput: based on case conclusion rates per courtroom from October 2024 to September 2025;
• “Higher” throughput: assumes a 10% increase in the average case throughput per court; and
• “Lower” throughput: assumes a 10% decrease in the average case throughput per court.

The three scenarios are summarised in Table 1.

Table 1. Prison population scenario variants.

The COVID-19 pandemic disrupted the justice system, with criminal court business temporarily restricted to essential business only[2]. Combined with increased court activity, this led to a backlog of trials. While court recovery resources have reduced the backlog from its peak, they remain high in High Court and Sheriff Solemn compared to pre-pandemic levels. This imbalance in the justice system makes it more difficult to estimate future case conclusion rates 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 October 2025 to March 2026. 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 November, December and January. To account for this, in the latest projections the assumptions for November and December 2025 are sampled from November and December 2024 and the assumptions for January 2026 are based on January 2025. Assumptions for the other months in the projection are sampled from the twelve-month period between October 2024 and September 2025.

Model Quality Assurance

At each update, the previous projections are compared with the actual prison population figures to determine the model’s continued suitability to produce reliable projections. Figures 14 and 15 show the projections from the microsimulation which were published in April 2025[3] and July 2025[4].

Even after accounting for a wide variety of uncertain dynamics in the system, Figure 14 indicates that the projected ranges for the remand, sentenced and total populations have aligned reasonably well with the observed population levels. Throughout the six-month projections the population remained within the confidence interval. At the end of the forecast period, the populations were close to the central conclusions scenario.

Figure 14. Prison population projections and the actual prison population beginning on 1st April 2025, based on courts and prison population data up to the end of March 2025. The actual population is shown as a series of black points up to 28th September 2025.

Figure 15 shows that the prison populations have also aligned with the latest projections published in August.

Figure 15. Prison population projections and actual prison population beginning on 1st July 2025, based on courts and prison population data up to the end of June 2025. The actual population is shown as a series of black points up to 1st October 2025.

To further assess the model, back-casting is used. This involves generating a retrospective projection using actual monthly court throughput data, thereby removing most assumption-based uncertainty. Any divergence between the back-cast and the actual figures may indicate technical limitations in the model.

The recent back-cast, presented in Figure 16, indicates that the actual prison population remained within the modelled range from April to September 2025.

Figure 16. The back-cast is based on remand arrival and courts throughput data from April 2025 to September 2025. The actual population is shown as a series of black points.

Microsimulation Model Limitations

The model relies on access to a large amount of high-quality, frequently updated data on court activity and prison populations. Some of this data can be resource intensive to obtain and process.

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