Defining a prolific offender
Source: data.gov.uk (2018-05-25)

Criminal Justice Statistics. The reports presents the main trends on the latest 12 months of activity in the criminal justice system (CJS) for England and Wales. For each process a brief description of the function is included with an explanation of some of the main procedures involved.

Prolific Offenders - Defining a Prolific Offender 
 
Introduction  
Prolific offenders who are they, why do we have so many, how do they become so prolific. These are 
the questions that are often asked. This is the first in a series of analytical papers which will explore 
this group of offenders, to answer these questions where possible. In the first instance we are 
seeking user’s views on the definition of a prolific offender.  
There is no common definition of a prolific offender and for the first time we try to define this group 
of offenders. For these analysis, a systematic approach has been adopted in trying to fully define a 
prolific offender. A prolific offender needs to be considered through various stages; their current age 
and the time available to offend (an offender aged 16 will have had a lot less time available to offend 
than a 50 year old). The proposed age group specific definitions are detailed below:–  
 
Methodology to define a prolific offender 
A basic definition means a prolific offender must have multiple offences resulting in convictions or 
cautions in their criminal career history. However, a threshold number of previous offences needs to 
be set when defining a prolific offender. In the Criminal Justice System Statistics quarterly 
publication a persistent offender is considered to be an offender with 8 or more previous convictions 
or cautions. In most analyses, an offender with 15 or more previous sanctions is considered a prolific 
offender.  
However, that definition is too simplistic to use as a single measure to define prolific offenders. 
Using a single value – such as 15 previous convictions or cautions – to define the cohort is likely to 
focus primarily on adults who have had most time to offend. For example, adult offenders 
DEFINITION 
Juvenile prolific offenders are defined as those who:  
 
are aged 10-17, and 
 
have 4 or more previous sanctions 
 
Young adult prolific offenders are defined as those who 
 
are aged 18-20, and 
 
have 8 or more previous sanctions, of which 4 or more were received whilst the offender 
was aged between 18 and 20.  
Where a young adult does not meet the criteria for a prolific young adult offender but received 4 or 
more previous sanctions as a juvenile, they will then form part of the juvenile prolific cohort. 
 
Adult prolific offenders are defined as those who:  
 
are aged 21 or older, and 
 
have 16 or more previous sanctions, of which 8 or more were received when the offender 
was aged 21 or over.  
Where an adult does not meet the criteria for a prolific adult offender but has 8 or more previous 
sanctions, 4 or more of which were received as a young adult, they will then form part of the young 
adult prolific cohort.  
 
Where an adult does not meet the criteria for a prolific adult nor a prolific young adult offender but 
has 4 or more previous sanctions as a juvenile, they will then form part of the juvenile prolific 
cohort. 

 
potentially could have committed many offences in their long criminal career whilst juvenile or 
young adult offenders will have had less time to offend in a shorter criminal career. A different 
threshold therefore needs to be considered by age group, as there are different sentencing 
guidelines and outcomes for juveniles and adults. Given these factors, it is more reasonable to have 
2 criteria and multiple thresholds for defining the term ‘prolific’ for three age groups of offenders:  
 
Criteria 1: Offending History – this applies to the minimum number of previous convictions 
or cautions needed for an offender to be considered prolific by the three age groups; 10 to 
17, 18 to 20 and 21 or over. 
 
Criteria 2: Criminal History Pattern – this criteria is applicable to the spread of an offender’s 
previous criminal history i.e. the age group when the majority of offending took place. An 
individual committing 15 offences as a juvenile and one as an adult could not be considered 
a prolific adult. 
 
Criteria 1: 
 
The MoJ’s extract of the Police National Computer identified just over 5 million individuals aged 10 
or older having received their last conviction or caution, dealt with by police force in England and 
Wales and British transport police, between 2000 and 2016. These offenders had an average of 4 
offences per offender. Using a basic assumption that a prolific offender should have more offences 
than the average offender, those with less than 4 offences were removed from the analysis leaving 
576,086 distinct offenders. Of these, 38,386 were aged 10-17, 46,011 were aged 18-20 and 491,689 
were aged 21 or over1.  
The next step is to define the number of previous convictions or cautions required to be considered 
a prolific offender for each age group; juvenile (aged 10-17), young adult (aged 18 to 20) and adult 
(aged 21 or over);  
 
The 38,386 juvenile offenders with 4 or more convictions or cautions had an average 
number of 4 offences per offender. Based on the assumption that a prolific should have 
more offences than the average, a juvenile prolific offender is defined as an offender who 
has been convited or cautioned on 5 separate occasions.  
 
Undertaking similar analysis for a prolific young adult offender, of the 46,011 young adults 
with 4 or more convictions or cations, they had an average of 8 offences. Therefore, a young 
adult would need to have been convicted or cautioned on 9 separate occasions to be 
considered a prolific offender. 
 
For adult offenders, of the 491,689 adult offenders with more than 4 sanctions, they had an 
average 16 offences each. Thus, an adult would need to have been convicted or cautioned 
on 17 separate occasions to be considered a prolific offender.  
 
 
 
 
                                                          
1 To be aware these figures are subject to change when a different PNC snapshot is used 

 
Criteria 2: Criminal history pattern 
The purpose of the second measure is to ensure that offenders who met the first measure are 
properly defined according to the spread overtime of their previous sanctions. For example, if an 
adult prolific offender received the majority of their sanctions when they were aged 18-20 or under, 
they would be reclassified as a young adult prolific offender (as this is the age at which they were 
most prolific). Similarly, a young adult prolific offender who had received most of their sanctions 
when they were aged 10-17 would be reclassified as a juvenile prolific offender. 
 
 
Definition of a prolific offender 
Earlier in this paper the proposed definition of a prolific offender was set out. Table 1 shows that of 
all prolific offenders, 20,690 (4%) were juveniles (aged 10 – 17), 41,462 (8%) were young adults 
(aged 18-20) and 429,374 (87%) were adults (21 or older) based on their last appearance in the 
criminal justice system.  Looking at the spread of previous convictions or cautions (shown in Working 
Examples 1, 2 and 3 above) four fifths of prolific offenders aged 18-20 and a quarter of those aged 
21 or older are classed as Juvenile prolific offenders. A quarter of prolific offenders aged 21 or older 
would be classed as a young adult prolific offender. 
 
 
 
 
 
Working Example 1 
“Juvenile prolific”: 
An offender is defined as a juvenile prolific if on their last appearance in the criminal justice system 
 
they were aged 10-17 and had 4 or more previous convictions or cautions (20,690 offenders); 
 
they were aged 18-20, had a total of 8 or more previous convictions or cautions, and had less than 4 
previous convictions or cautions when aged 18-20 (33,352 offenders); and 
 
they were aged 21 or older, had less than 16 previous convictions or cautions, had less than 4 previous 
convictions or cautions when aged 18-20 and had 4 or more previous convictions or cautions when 
aged 10-17 (107,056). 
Working Example 2 
“Young adult prolific”: 
An offender is defined as a young adult prolific if on their last appearance in the criminal justice system 
 
they were aged 18-20, had a total of 8 or more previous convictions or cautions, and had at least 4 
previous convictions or caution when aged 18-20 (8,109 offenders); and 
 
they were aged 21 or older, had a total of 16 or more previous convictions or cautions, had less than 8 
previous convictions or cautions when aged 21 or older, and had 4 or more previous convictions or 
cautions when aged between 18 and 20 (110,373 offenders). 
Working Example 3 
“Adult prolific”: 
An offender is defined as an adult prolific if on the last appearance in the criminal justice system 
 
they were aged 21 or older, had a  total of 16 or more previous convictions or cautions, and had 8 or 
more previous convictions or cautions when aged 21 or older (211,945 offenders). 

 
 
Table 1: Number of offenders in each prolific group by age, gender and ethnic appearance, 2000-2016 
Number
%
Number
%
Number
%
Number 
%
Age
10-17
20,690
12.8%
648,378
12.9%
18-20
33,352
20.7%
8,109
6.8%
590,767
11.7%
>=21
107,056
66.5%
110,373
93.2%
211,945
100.0%
3,805,980
75.4%
Characteristic
Juvenile Prolific
Young Adult Prolific
Adult Prolific
Non-prolific
 
 
The following flow diagram gives a visual representation as to how the prolific offender cohort has 
been selected. 

 
Figure 1: Flow diagram of prolific cohort selection 
Individual offenders recorded on the 
PNC between 2000 - 2016 
(5,207,744 )
All offenders with a total number of convictions
/ cautions  4 or more
(576,086 )
Offenders aged 
10 - 17 
(38,386 )
Offenders aged 
18 - 20 
(46,011 )
Offenders aged 
21 or older
(491,689 )
Total number of previous convictions 
/ cautions >=4 ?
Yes(20,690)
No (17,696)
Total number of previous convictions /
cautions 
>=8?
Is the number of previous convictions 
/ cautions >= 4 when aged 18 -20? 
No (37,902)
Yes (8,109)
Total number of previous 
convictions / cautions
>=16?
Is the number of previous 
convictions / cautions >= 8
when aged 21 or older?
Is the number of previous 
convictions / cautions >= 4 
when aged 10 -17 
Yes (33,352)
No (4,550)
Prolific Juveniles (161,098)
Prolific young adult (118,482)
Prolific adult (211,945 )
No (279,744)
Yes (211,945)
Is the number of previous 
convictions / cautions >= 4 when 
aged 18 -20? 
No (169,371)
Yes (110,373)
Is the number of previous 
convictions / cautions >= 4 
when aged 10 -17 
No (62,315)
Yes (107,056)

 
Feedback and Questions 
We would welcome any feedback from users on the proposed definition for a prolific offender. 
Questions 
1. Do users agree that a prolific offender should be defined on an age group basis, i.e. if the 
majority of the offending took place as a juvenile, then that is the age group for which they 
are defined as a prolific offender irrespective of their age at last offence? 
2. Do users agree that to be prolific an offender must have committed more offences than 
the average of the age group they are in? 
3. Do users agree with the criteria used to define prolific offenders? 
4. Would users find a more in depth analysis of prolific offenders based on this definition 
useful? 
5. If you disagree with the proposed definition, please provide detail as to what you believe 
makes a prolific offender. 
 
Please send your feedback to xxx@xxx.xx by Friday 15th December 2017. 
 
Subject to any feedback received with regards to the proposed prolific offender definition we will be 
publishing offence and outcomes analysis of a prolific offender in the February 2019 publication.