7. Technical annex

Terminology definitions

This section includes explanations of modelling practice and terms to describe the level and range for the projections of the total, sentenced and remand populations.

Microsimulation Scenario definitions

As in the first publication of June 2023, only low, central, and high court throughput scenarios are reported. It is assumed that remand arrivals will be similar to the level seen over the 12 months leading up to February 2025. These scenarios use common underpinning data on court case progression and the prison population, from SCTS and SPS, respectively. The scenarios are distinct from each other in that they rely on differing assumptions about how future levels of court conclusions might change.

The microsimulation prison population modelling methodology requires each scenario to be simulated repeatedly dozens of times so that the combined output yields confidence intervals for the total, sentenced and remand population for each scenario. The full projection includes the 95% range intervals for all three scenarios. These are overlapping in Figures 1, 14 and 16, such that it is not possible to easily identify individual scenarios. The values denoting the upper and lower estimates of the populations in the scenarios in table 2 represent the corresponding outer bounds of the 95% range interval for the widest ranging scenarios in the projection.

Court throughput rate scenario definitions

The central court throughput rate scenario variant assumes that the conclusions per courtroom will be similar over the next few months to the typical level over the year leading up to the 1st April 2025 data cut-off date. The "high" scenario assumes the average and range of case conclusions per court per day will be around 10% greater than it has been over the same period, and the "low" scenario assumes that the average case throughput per court will be 10% smaller. A rolling sampling period means that for each edition of the projections the model uses data on case conclusion rates from the last year, thereby accounting for the most recent trends in each successive update to the modelling.

Short Term Prisoner Automatic Early Release 2025

The Prisoners (Early Release) (Scotland) Act changed the release point for some offenders serving sentences of less than four years (short term prisoners) from 50% of their sentence to 40%. There was no change to the release point for prisoners serving sentences for domestic abuse or sexual offences, who continue to be released at 50%. From March 2025, all eligible short-term prisoners will be released at the 40% point of their sentence[1]. 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.