Prison Population Projections 2014 – 2020 
Appendix B: Detail of models, scenarios and assumptions 
The updated modelling approach 
The prison projections form part of the Ministry of Justice’s wider work to 
develop a consistent and coherent suite of models of the criminal courts and 
offender management, driven by common projections of demand for the 
Ministry of Justice’s services.  
The prisons model used to generate the 2014 projections has not changed 
substantially from that used in the 2013 projections. As in the 2013 projections 
custodial sentence lengths used in the model are disaggregated by gender, 
age of the offender and offence type.  The total time to be served in prison by 
projected future prisoners is assigned by matching their gender and age 
characteristics to relevant distributions of (i) custodial sentence lengths and (ii) 
the percentage of custodial sentence served. These distributions are derived 
from data for the period October 2013 to April 2014.  This allows us to: 
 understand the Criminal Justice System factors which contribute to 
change in the prison population, including sentences lengths issued, 
the percentage of sentence served in custody, trial court and 
sentencing court changes, or shifts in the demographic characteristics 
of defendants;  
 model the impact on the prison population of specific Ministry of 
Justice and other Criminal Justice Agency policy changes; and 
 quantify the impact of uncertainty around the time a defendant serves 
in prison on the prison population. 
  
Overview of the modelling approach 
Central to the modelling approach is the Prison Population Stock-Flow model. 
Projections of future custodial convictions are fed into this model and outputs 
are adjusted to account for the impact of changes in legislation and process 
on the prison population, as shown in Figure B1, and described below. 
 
 
 
 
 
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