Scottish prison population projections: December 2024

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

In the first publication of June 2023, only low, central and high court throughput scenarios were reported. Each new edition of this publication since November 2023 has featured nine different scenario variants. Since the start of 2024 remand arrivals have been consistently elevated therefore this edition of the Scottish Prison Population projections reverts to the use of the three scenarios in the first issue of the publication, with the assumption remand arrivals will remain elevated in the first quarter of 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 and remand arrivals 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 13 and 15, 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 variants assume 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 November 2024 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.