Introduction to Prison Population Projections

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 July 2024 to June 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 Sherrif 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 July 2025 to December 2025. Remand arrivals are sampled from the twelve-month period between July 2024 and June 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. Figure 14 shows the projections from the microsimulation which were published in April 2025[3]. 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. To date, the actual figures have most closely tracked the higher court case conclusion scenario.

Figure 14. Prison population projections and 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 30th July 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 15, indicates that the actual prison population remained within the modelled range from January to June 2025. However, the estimated sentenced population tended to be higher than the actual population from April 2025 onwards, following the implementation of tranche 3 of STP40 early release.

One of the causes of this discrepancy is that the initial population for the projection is based on the population on 1st January 2025, prior to the STP40 changes. The release dates for some members in that population were altered under the new legislation. Due to the structure and function of the microsimulation model, it is not possible to identify all those who would’ve been eligible to be released earlier, leading to an overestimation of the sentenced population.

This limitation does not affect the current projections, which use the 1st July population as the starting point. All release dates in this population reflect the STP40 legislation.

In contrast, the remand population – unaffected by the STP40 legislation – shows strong alignment between the back-cast and the actual figures from January to June 2025.

Figure 15. The back-cast is based on remand arrival and courts throughput data from January 2025 to July 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.