Prison population projections: November 2023 edition

4 Introduction to Prison Population Projections

This section provides an overview of the assumptions and scenarios developed to project the prison population and explains how the model’s previous projections are validated for robustness against actual prison population figures in recent months. The first issue of the Scottish prison population projections^[19] provides a more detailed overview on the modelling technique used.

Model Assumptions and Scenarios

The scenarios on which the projections are based rely on a variety of assumptions about how the rate of transitions into and out of the prison population might change. These can be influenced by trends and planned changes to the court system, including: court capacity changes, increased court conclusions rate, increased remand arrivals, remand/bail mix and 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.

Due to the level of the flows which have contributed to growth in the sentenced and remand populations since early 2023, and the abrupt seasonal population changes which are likely around the end of December 2023 and the beginning of January 2024^[20], additional scenarios have been developed. In the previous publication only low, central and high court throughput scenarios were considered. In addition to these three court throughput scenarios, there are now six new scenarios featuring varying remand arrivals (low, central and high) combined with court throughput variation. A full breakdown of all nine scenarios is given in table 1. These additional scenarios are intended to account for uncertainty in future remand arrivals, especially in early 2024. This is because unlike during 2022 when it decreased, the remand population has increased over the course of 2023 so far. Additionally, case registrations in Sheriff Summary and Sheriff Solemn were higher in Q1 2023/24 than during Q1 2022/23⁴.

Three variations of remand arrival have been used for the scenarios: central, higher and lower. The central remand arrivals scenario assumes remand arrivals will be similar over the next few months to what they were between October 2022 and September 2023. The higher remand arrivals scenario assumes the rate will be similar to the levels from March 2023 to August 2023, and the lower scenario assumes remand arrivals around the lower levels between October 2022 and March 2023.

Furthermore, to help with understanding how sensitive the prison population may be to variations in court case conclusion rate, three variants have again been included in the modelling - central, higher, and lower. The “central” court throughput scenario variant assumes that the conclusion rate per court will be similar over the next few months as it has been between October 2022 to September 2023. The “higher” scenario assumes the average case throughput per court will be slightly greater than it has been over the same period, and the “lower” scenario assumes that the average case throughput per court will be slightly smaller. The nine scenario variants shown in table 1 include all the possible combinations of the above variations of court throughput and remand arrivals.

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 2023 to March 2024.

A review of previous trends shows that a general seasonal pattern tends to occur at the end of the year. This results in irregular court throughput and remand arrivals during the months of December and January. To account for this, in the latest projections the assumptions for the month of December 2023 are sampled from December 2022 and January 2024 from January 2023.

Model Quality Assurance

The modelling scenarios from the previous publication can be compared against the actual population data to check the accuracy of the projections and confirm the model’s suitability for providing reliable projections. Figure 12 shows the April 2023 central, high and low court throughput projections from the microsimulation, based on prison population and courts data up to the end of March 2023. Even after accounting for a wide variety of uncertain dynamics in the system, it may be seen in Figure 12 that the projected levels of remand, sentenced and total populations reasonably accurately aligned with the actual population.

To check the model further, back-casting is used to retrospectively compare the actual prison population for the past few months against a projection generated by the model based on actual monthly court throughput data. The back-cast eliminates uncertainty about the majority of the assumptions, so if there was a difference between the back-cast and the actuals it may indicate technical deficiencies in the model. The recent back-cast projections presented in Figure 13, show that the back-cast from April 2023 to September 2023 is largely accurate.

Microsimulation Model Limitations

The model relies on the availability of a large amount of frequently refreshed high-quality data about court activity and prison populations, some of which can be resource intensive to obtain and process.

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