Prison Population Projections 2014 – 2020 
3a) Producing prison population projections 
Prison population projections are produced using the Prison Population Stock-
Flow Model. The principal sub-populations in prison – determinate sentence, 
life sentence, imprisonment for public protection (IPP) and remand – are 
modelled using stock-flow structures based on the generic structure shown in 
Figure B2. The stock-flow structures model the flow of offenders into and out 
of prison and count the resulting prison population at the end of each month. 
Figure B2: Generic stock-flow structure in the Prison Population Stock-Flow 
Model 
Prison Population
Receptions / Pre-tariff ends Discharges
Average Time Served
Prison Population
Receptions / Pre-tariff ends Discharges
Average Time Served  
For the determinate population, the monthly inflows to prison are based on the 
custodial convictions projections described above. These custodial convictions 
include offenders that may already be serving a sentence for a previous crime 
or those who would serve their whole custodial sentence on remand, meaning 
that they would not be a new reception to prison. To convert from custodial 
convictions to prison receptions we apply a conversion ratio derived from the 
historical proportions of custodial convictions to prison receptions for each 
sub-population averaged over the last twelve months of historical data (April 
2013 to March 2014 inclusive).  
Monthly outflows for the determinate population are based on observed 
custodial sentence lengths and the observed percentage of sentence length 
served taken from October 2013 to April 2014. Each projected offender that 
enters the model is given a custodial sentence length that is randomly 
selected from the relevant distribution. These distributions are populated with 
custodial sentence lengths from actual offender receptions who share the 
same characteristics of offence, gender and age group in the observed time 
period. The percent of custodial sentence length served is derived in the same 
manner, except that the observed distribution is made up of discharged 
offenders further disaggregated by custodial sentence length band. 
For offenders who receive the new EDS sentence an adjustment is made to 
the percent of custodial length served to reflect that these offenders will spend 
a greater proportion of their sentence in custody than standard determinate 
sentenced offenders discharged to date. 
Projected prison receptions are sub-divided by age category (Juvenile, Young 
Adult, Adult) with the exact age of the offender attributed in the same manner 
as the custodial sentence lengths. This allows the model to explicitly age the 
offenders whilst in prison (e.g. move from Juvenile to Young Adult categories). 
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