Scottish prison population statistics technical manual

Information on data presented in the Scottish Prison Population Statistics reports from 2019-20 onwards.


Analytical factors and measurements

Measurements

 

Factors

Measurements

This section details the different measures made of the prison population in our statistical releases.

Individuals

A count of individuals indicates the number of unique individuals identified in each subgroup or combination of subgroups in the population in the course of an Analytical Period. Individuals are differentiated by their prisoner number.

A result of counting individuals in this way is that, in cases where an individual can occupy more than one subgroup in a given analytical period, a sum down a column or across a row in an analytical table may include some people more than once. For example, where a person spends time in multiple prisons in the course of an analytical period, they will be counted once in each prison they stay in. Adding together the individuals identified in all prisons will therefore exceed the total number of people imprisoned in that period.

Table 2.1 lists the analytical factors where a person may occupy multiple categories over the course of given period.

Table 2.1: Categories where an individual may have multiple values over the course of an analytical period

All Individual measurements relating to:

Measurements of Individuals Arriving:

Measurements of Individuals Departing:

Legal Status

Of No Fixed Abode

Journey Type

Index Offence Group

Local Authority

Journey Type-2

Index Offence

SIMD 2016 Decile Group

 

Index Sentence

 

 

Prison Establishment

 

 

Average daily population

The average daily population (ADP) over a given Analytical Period across the prison estate is the total number of prisoner days served, divided by the number of days in that period.

A person’s occupancy time within an Analytical Period – termed prisoner days – provides a weight for each prisoner in that analytical period.

When calculating the length of a Stint, some edge cases exist: for example when a prisoner enters and leaves prison on the same day. Since the time of entry and exit are not recorded in these data, this would count as zero prisoner days, and therefore zero weight. To account for this, such stints are assigned a value of 0.25 prisoner days (6 hours) by default.

Note: ADP measurements tend to obfuscate some of the differences between populations. For example, one population where a large proportion of a small number of individuals spend the whole year in prison is indistinguishable in this measurement from another population where a greater number of individuals cycles through prison for a short period of time. This effect is illustrated in Figure 6, where a single prisoner present for 6 consecutive days provides the same average daily population as 6 prisoners present for one day each.

Figure 6: Average Daily Population equivalence between many short-term and few long-term prisoners

Average Daily Population equivalence between many short-term and few long-term prisoners

Arrival

An arrival is when a period of imprisonment begins, and is only counted as such if the arrival occurred on a separate day from any previous departure.

These may be subdivided based on the Legal Status on the first day of an Occupancy Period, e.g. “Arrival to Sentenced”

Counting the number of Individuals arriving provides the number of unique individuals arriving in each subgroup in the analysis.

Departure

Departure is counted when an individual is absent from prison for one or more nights.

These may be subdivided based on the Legal Status on the last day of an Occupancy Period, e.g. “Departure from Remand”.

Counting the number of Individuals departing provides the number of unique Individuals departing in each subgroup in the analysis.

Status transition

A status transition is counted when an individual moves from a period on remand to a period serving a sentence without leaving custody over night. They may be counted alongside Arrivals to Sentenced status or Departures from Remand.

Counting the number of Individuals transitioning provides the number of unique individuals transitioning to or from remand in each subgroup in the analysis.

Time Served (Continuous)

All measurements of time served are based on continuous periods in SPS custody. Most commonly this measurement is reported as a factor in banded categories of Time to Departure or Time on Remand. Measurements are taken at Departure or (in the case of time on remand) upon Status Transition.

The measurements provided in prison population statistics reflect both the construction of the database – based on Occupancy Periods – and the distribution of the data.

Time served (in general) has a “long tail” distribution, meaning there are a few individuals in each cohort with very long periods spent in custody, while most will spend much shorter periods in custody overall. As a result of this distribution, the median is used as the more representative measure of central tendency (as opposed to the mean).

Following feedback from users and stakeholders, and to provide better information about the wider distribution, the 90th percentile is also provided in most cases to indicate the minimum stay for the 10% of the population staying the longest.

Comparator population rates

Throughout the statistical releases, comparisons are made between the prison population and the general population in Scotland.  These are the sources for those comparisons:

Populations

The overall Scottish population, and local authority populations, are estimated by National Records of Scotland each year. These provide the basis for comparison and for population rates by local authority.

Where population rates are presented for countries other than Scotland they are drawn from external sources which should be referred to directly.

Ethnicity

The base population by ethnicity for 2011-12 is drawn from the 2011 Census. For subsequent years, ethnic group sizes are drawn from the pooled sample of Scottish Surveys Core Questions (SSCQ). Due to the SSCQ's methodology, population rates are based only on populations aged 16 or over. Using SSCQ data in this way also makes it possible to control for the increasing age (into adulthood) of those youngest minority ethnic populations when the census was taken in 2011.

The SSCQ estimates of ethnic group population sizes have an associated margin of error. The calculation of this margin of error is explained in the SSCQ technical report (section 1.3). The upper and lower confidence bounds provided with the prisoner population rate statistics indicate a 95% confidence that the true population rate lies between those two values. This is like saying if the SSCQ were performed 100 times, the estimated population of the different ethnic groups in Scotland would fall within those bounds in 95 of the 100 runs.

Data from the SSCQ is only available up to the calendar year 2019. For subsequent years these data have been reused. This will be subject to revision when the results of the census are made available.

Factors

Prisoner age

Prisoner age is calculated based on dates of birth and the details of the Analytical Period.

In the first publication from this source, prisoner ages were based solely on the middle date of the analytical period, i.e. 1st October for each financial year. This has since been revised. Calculating ages in this way meant that a number of young people arriving after 1st October who had birthdays between that date and their arrival date were incorrectly identified as being under 16 years of age.

Prisoner ages are calculated as at 1st October where they arrived prior to that date (including those arriving in a previous analytical period). For those arriving after the 1st October, their age on arrival is used.

Age Bands

Age bands are based on Prisoner Age and grouped into five-year age bands except for the following bespoke categories:

  • Under 16
  • 16-17
  • 18-20
  • 21-22
  • 23-24
  • 75 and over

Gender

The collection of Prisoner Gender is determined by SPS recording policy.

Age groups

Division by age into young people (under 21) and adults.

Prisoner Age in this case is banded to separate individuals classified as Young Offenders (20 and under).

Age-Gender

Age-Gender are based on a combination of Gender and Age Groups.

Marital status

Self-reported relationship status.

Where the number of individuals in a category is small it is necessary to combine these groups for the purposes of reporting. This is likely to mask variations within aggregated groups.

The combined categories are listed in Table 2.2.

Table 2.2: Self-Reported Marital Status Categories

Reporting Category

Source Category

Single

Single

Married/Partnered

Married

Common Law

Civil Partnership

Divorced/Separated

Divorced

Separated

Dissolved Civil Partnership

Surviving Partner

Widowed

Surviving civil partner

Ethnicity

Self-reported ethnic identity.

Categories are provided by the harmonised principles developed for the census 2011 by the Government Statistical Service and National Records of Scotland. Where the number of individuals in a category is small it is necessary to combine these groups for the purposes of reporting. This is likely to mask variations within aggregated groups.

The combined categories are listed in Table 2.3.

Table 2.3: Self-Reported Ethnicity Categories

Reporting Category

Source Category

White

Scottish, English, Welsh, Northern Irish or British

Irish

Gypsy or Irish Traveler

Polish

Any other White background

Asian, Asian Scottish or Asian British

Pakistani, Scottish Pakistani or British Pakistani

Indian, Scottish Indian or British Indian

Bangladeshi, Scottish Bangladeshi or British Bangladeshi

Chinese, Scottish Chinese or British Chinese

Other

African, Caribbean or Black

African, Scottish African or British African

Other African Groups

Caribbean, Scottish Caribbean or British Caribbean

Black Other

Mixed or multiple ethnic groups

Mixed White and Asian

Mixed White and Black African

Mixed White and Black Caribbean

Mixed Other

Other ethnic groups

Arab, Scottish Arab or British Arab

Middle Eastern Other

Other Ethnic Origin

Disability

Self-reported disability.

Sexual Orientation

Self-reported sexual orientation.

Where the number of individuals in a category is small it is necessary to combine these groups for the purposes of reporting. This is likely to mask variations within aggregated groups.

Non-heterosexual categories are combined into a single category: Lesbian, Gay, Bisexual and Other (“LGB & Other”). Those declining to respond are retained separately.

Armed forces

Self-reported history of employment in the armed forces. These data are provided as the two-fold variable "Veterans" indicating past service in any of the UK armed forces, and as the four-fold variable indicating which of the services the individual served in.

Service Number is collected and checked by SPS where possible, but is not extracted by Scottish Government analysts.

Legal status

One of Untried (U), Convicted Awaiting Sentencing (A) or Sentenced (S).

Convicted Awaiting Sentencing is sometimes shortened to “CAS”.

In the 2-fold factor, Untried and CAS together are termed “Remand” (R), as distinct from "Sentenced" (S).

Time within an Occupancy Period is subdivided into different statuses based on the Legal Status Hierarchy.

Index offence

The offence category for which the longest sentence has been received over a period.

For prisoners on remand, the index offence is determined based on the average sentence associated with offence types from SG Criminal Proceedings.

For further information, see Index Offence derivation.

Index offence group

The Index Offence for which the longest sentence has been received over a period, grouped into higher-level categories. Groupings are provided in Table 2.4.

For further information, see Index Offence derivation.

Note: Offences in categories outside of group 1 (Crimes of Violence) may entail violence against their victims. Common Assault, for example, covers a wide range of behaviours that can include lower-level physical violence.

Table 2.4: Index Offence Higher-level Groupings

Group Group Name SG50 Labels
1 Non-sexual crimes of violence Murder and culpable homicide
    Causing death by driving dangerously
    Serious assault and attempted murder
    Common assault
    Robbery
    Domestic Abuse (Scotland) Act 2018
    Other non-sexual violence
2 Sexual crimes Rape & attempted rape
    Sexual assault
    Causing to view sexual activity or images
    Communicating indecently
    Threatening to or disclosing intimate images
    Indecent photos of children
    Crimes associated with prostitution
    Other sexual crimes
3 Crimes of dishonesty Housebreaking
    Theft by opening lockfast places
    Theft from a motor vehicle
    Theft of a motor vehicle
    Shoplifting
    Other theft
    Fraud
    Other dishonesty
4 Damage and reckless behaviour Fire-raising
    Vandalism
    Reckless conduct
5 Crimes against society Crimes against public justice
    Weapons possession (not used)
    Drugs - Supply
    Drugs - Possession
    Other crimes against society
6 Antisocial offences Threatening and abusive behaviour
    Racially aggravated conduct
    Drunkenness and other disorderly conduct
    Urinating etc.
    Community and public order offences
7 Miscellaneous offences Coronavirus restrictions
    Environmental offences
    Licensing offences
    Wildlife offences
    Other misc. offences
8 Road traffic offences Dangerous and careless driving
    Driving under the influence
    Speeding
    Unlawful use of vehicle
    Vehicle defect offences
    Seat belt offences
    Mobile phone offences
    Other road traffic offences

Index sentence

The longest single sentence being served during a period in custody.

Categories in banded index sentence length analysis are provide in Table 2.5.

For further information, see Index Sentence derivation.

Table 2.5: Index Sentence bands

Index Sentence Band

Low (days)

High

Description

Untried

-

-

Remand

CAS

-

-

Remand

0<=3 months

1

92

 

3<=6 months

93

184

 

6<=12 months

185

366

 

12<=18 months

367

548

 

18<=24 months

549

731

 

2<=4 yrs

732

1461

 

4 < 10 yrs

1462

3651

 

10+ yrs

3652

 

Life/OLR

-

-

Any life sentence supersedes any determinate sentence or recall from the date it was first passed

Recall

-

-

Recalls supersede any determinate sentence for categorisation

Unknown

-

-

While the occupancy period has an associated sentenced warrant, the information on sentence duration is missing

(Missing)

-

-

There is no associated sentence information associated with this occupancy period

Prison establishment

CellWise data covers a period of transition in the prison estate, and includes historic information about populations in prisons that have since closed.

Prisons, or “Establishments”, included in the cellWise data are listed in Table 2.6 along with their approximate opening and closing dates.

The small capacity of Community Custody Units (CCUs) Bella and Lilias has led us to combine the two facilities in our statistical outputs to maintain the anonymity of the people staying there.

Table 2.6: Prisons in the scope of Scottish Prison Population Statistics

Prison

Establishment

Opened

Closed

HMP YOI

Aberdeen

1890

2014

HMP

Addiewell

2008

 

HMP

Barlinnie

1880

 

HMP YOI

Cornton Vale

1975

2023

HMP

Dumfries

1863

 

HMP YOI

Edinburgh

1920

 

HMP

Glenochil

1966

 

HMP YOI

Grampian

2014

 

HMP

Greenock

1907

 

HMP

Inverness

1902

 

HMP

Kilmarnock

1999

 

HMP

Low Moss

2012

 

HMP

Open Estate (Castle Huntly)

1946

 

HMP

Perth

1812

 

HMP

Peterhead

1888

2013

HMP YOI

Polmont

1911

 

HMP

Shotts

2012

 

HMP YOI Stirling 2023  
CCU Bella 2022  
CCU Lilias 2022  

Of no fixed abode

Self-reported status. On arrival the individual is unable to, or declines to, provide a home address.

As with other geographical data, this measure is only available for Arrivals to custody in each Analytical Period.

Local authority

Local authority area of a prisoner's home address on arrival or, for those of no fixed abode, their local authority of residence.

As with other geographical data, this measure is only available for Arrivals to custody in each Analytical Period.

SIMD 2016 Decile Group

Area deprivation of home address, based on the postcode of those with a known address in the Geographical Data. As with other geographical data, this measure is only available for Arrivals to custody in each Analytical Period.

Scottish Index of Multiple Deprivation provides a typology of datazones in Scotland. This index is broken down into decile groups, with the first group representing the most deprived 10% of datazone areas and the 10th group representing the least deprived 10% of areas.

Some prisoners are excluded from this analysis:

  • Unknown 1 - No match and NFA: The prisoner is of no fixed abode.
  • Unknown 2 - No/partial postcode: the postcode data is incomplete and therefore cannot be matched to a datazone
  • Unknown 3 – No match: The information provided in the postcode field cannot otherwise be matched to the datazone lookup file.

Journey type

Legal Status stages in an individual's journey during a period in prison: Untried (U), Convicted Awaiting Sentencing (A) or Sentenced (S).

Note: the simplified journey types presented here mask the ambiguity in legal status arising as a result of the one-directional Legal Status Hierarchy inherent in the cellwise data construction.

Journey type 2

Legal Status stages in an individual's journey during a period in prison: Remand (R), or Sentenced (S).

Note: the simplified journey types presented here mask the ambiguity in legal status arising as a result of the one-directional. Legal Status Hierarchy inherent in the cellwise data construction.

Liberation type

Table 2.7 lists the liberation descriptions provided by the prisoner record system, and the way they have been combined into higher level categories.

Some liberation types (e.g. “Sentence Served”) would be expected to follow a period in custody with the Sentenced Legal Status. However, due in part to the one-directional Legal Status Hierarchy inherent in the cellwise data construction, liberation descriptions do not always tally with the apparent legal status at the end of a listed Journey Type. For reference, in Table 2.7 we also provide the expected legal status of individuals leaving prison with different liberation types.

Table 2.7: Liberation types

Expected Status On Liberation

Summary Liberation Type

Full Liberation Description

Sentenced

Sentence Served

Lib Sentence Expired

On licence/Parole/SRO/HDC

Home Detention Curfew

Lib On Licence

Lib On Parole

Lib To Supervised Release Order

Appeal/Pardon

Lib On Appeal

Pardon

Fine Paid

Lib Fine Paid

Remand

Bailed

To Bail

Court/PF

Court

Lib From Court

Procurator Fiscal

Ambiguous

Immigration/Deportation

Lib Immigration Author

Lib Dungavel Detention Centre

Deported

Repatriation

Lib To Early Removal Scheme

Other

Deceased

Lib To Mental Hosp

Lib To List D

Local Authority

Discretionary Early Release

Gender Reassignment

Lib To Interlib

Police Interview


People who have died in prison are counted in the “Other” category in Table 2.7. Deaths in custody are the subject of Scottish Prison Service reporting, available on the SPS website.

Time to Departure

Overall time in custody for a continuous occupancy period.

Upon Departure during an Analytical Period, we can determine the total continuous time spent by a prisoner in custody. Banding prisoner days from arrival to departure provides the categories for this variable, as follows:

  • 0<=3 months - less than or equal to three months
  • 3<=6 months - greater than three months and less than or equal to six months
  • 6<=12 months - greater than six months and less than or equal to twelve months
  • 12<=18 months - greater than twelve months and less than or equal to eighteen months
  • 18<=24 months - greater than eighteen months and less than or equal to two years
  • 2<=4 yrs - greater than two months and less than or equal to four years
  • 4 + yrs - greater than four years

This information should not be taken as an indicator of time served on a specific sentence. Prisoners may leave custody during their sentence for a night or more for a variety of reasons, thereafter returning to complete their time in custody.

Time on remand

Time spent with a remand status (untried/convicted awaiting sentence) before the first intersecting sentencing warrant.

Upon Departure from a remand Legal Status, or upon Status Transition to the sentenced population, we can determine the total continuous time spent by a prisoner on remand. Banding prisoner days from arrival to departure or transition provides the categories for this variable, as follows:

  • 1 day or less
  • 2-7 days
  • <=2 weeks
  • 3 weeks
  • 4 weeks
  • 5 weeks
  • 6 weeks
  • 7 weeks
  • 8 weeks
  • 9 weeks
  • 10 weeks
  • 71-140 days
  • > 140 days

Financial Year

Financial years run from the 1st April to the 31st March.

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