Technical Annex

Terminology definitions

This section includes definitions and explanations of modelling practices and terminology used to describe the level and range of projections for the total, sentenced and remand prison populations.

Microsimulation scenario definitions

As in the first publication of June 2023, three scenario variants are reported: “low”, “central”, and “high” court throughput. It is assumed that remand arrivals will remain similar to the levels observed in the 12 months leading up to July 2025.

All scenarios use common underpinning data on court case progression and the prison population, sourced from the SCTS and the Scottish Prison Service, respectively. The scenarios differ only in their assumptions about how future levels of court case conclusion rates may evolve.

The microsimulation methodology requires each scenario to be simulated repeatedly – dozens of times – to generate confidence intervals for the total, sentenced and remand populations. The full projection includes the 95% confidence intervals for all three scenarios. These intervals are shown in Figures 1, 14 and 16, and are overlapping, such that it is not possible to easily identify individual scenarios visually.

The upper and lower estimates of the populations in Table 2 represent the outer limits of the 95% confidence intervals for the most divergent scenarios.

Court throughput rate scenario definitions

The “central” scenario assumes that the average case throughput per court will remain similar to the average observed over the year leading up to the 1st July 2025 data cut-off.

The "high" scenario assumes that the average case throughput per court will be 10% greater than observed.

The "low" scenario assumes that the average case throughput per court will be 10% lower.

A rolling sampling period is used, meaning each edition of the projections incorporates case conclusion data from the most recent twelve-month period. This ensures that the model reflects current trends in each successive update.

Short-Term Prisoner Automatic Early Release 2025

The Prisoners (Early Release) (Scotland) Act changed the release point for some individuals serving sentences of less than four years from 50% to 40% of their sentence. There was no change to the release point for individuals convicted of domestic abuse or sexual offences, who continue to be released at 50%.

From March 2025, all eligible short-term prisoners have been released at the 40% point of their sentence. The projections in this publication account for the ongoing reduction in the sentenced population, by adjusting release points for modelled sentenced prisoners in line with the Act.