Prison statistics and population projections Scotland: 2011-12

This publication shows data up to 2011-12 on Scottish prison population levels and characteristics, receptions to/liberations from Scottish prisons, and international comparisons. This year it includes prison population projections to 2020-21 which were previously published separately, as well as additional background information and analyses.

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Annex C Projection methodology

Projection method and time served factors

Projection method: December 2011 Table C.1

Population Projection method
Remand Fixed proportion of direct sentenced receptions
Sentenced adults
Fine defaulters Receptions held constant
Less than 6 months Full reception projection
6 months up to 18 months Full reception projection
18 months up to 2 years Full reception projection
2 years up to 4 years Full reception projection
4 years and over, excluding Life Full reception projection
Life prisoners Population projected directly using linear regression
Sentenced young offenders
Fine defaulters Receptions held constant
Less than 6 months Full reception projection
6 months up to 18 months Full reception projection
18 months up to 4 years Full reception projection
4 years and over, excluding Life Full reception projection
Life/s205/s206 prisoners Population projected directly using linear regression
Persons recalled from supervision/licence Fixed proportion of long-term population
Others Population held constant

Note: Since 2004, the population of prisoners sentenced to life has been projected directly. This was due to changes in release policy for this group, with lifers being currently eligible for parole once the punishment part of their sentence has been served. This has resulted in larger numbers of long-serving lifers being released than had previously been the case, causing the projection method for lifer receptions to provide unrealistic estimates for the lifer population. The population of lifers is currently projected directly using linear regression over the period 1995 to 2011.

Time served factors: December 2011 Table C.2

  Time served factors
(days)
Remand 27
Sentenced adults
Fine defaulters 3
Less than 6 months 27
6 months up to 18 months 93
18 months up to 2 years 209
2 years up to 4 years 361
4 years and over, excluding Life 1,150
Life prisoners n/a
Sentenced young offenders
Fine defaulters 2
Less than 6 months 22
6 months up to 18 months 88
18 months up to 4 years 253
4 years and over, excluding Life 593
Life/s205/s206 prisoners n/a
Persons recalled from supervision/licence n/a
Others n/a

Note: Time served factors are estimated by dividing the average daily population by the number of receptions. This represents the time spent in custody as a sentenced prisoner, excluding time spent on remand or home detention curfew. For young offenders, this represents the time spent in custody as a young offender.

Accuracy of projections

82. Projections are based on past trends, and therefore periods of relatively rapid change may result in a loss of accuracy in the short to medium term. Chart C.1 shows the historic trajectory of previous projections compared with the actual annual population. A point of interest is that projections which may have been relatively inaccurate in the short term can turn out to be reasonably accurate in the longer term as the population evolution enters a more stable state.

Chart C.1 Historical accuracy of past projections

Chart C.1 Historical accuracy of past projections

83. The prison population currently appears to be in a transition phase similar to that observed in the mid to late 90s, with periods of marked increase followed by short-term drops. This is likely to result in some fluctuation in the short-term accuracy of the population projections, as measured by the difference between the observed and predicted population for the first full year projected (Table C.3). However, it can be noted that the projections over the past few years all indicate long-term average annual increases in the population in the order of 2 per cent.

Accuracy of past projections Table C.3

Year Difference between observed and predicted % difference
1996-97 93 2
1997-98 159 3
1998-99 -321 -5
1999-00 -225 -4
2000-01 -317 -5
2001-02 182 3
2002-03 53 1
2003-04 3  *
2004-05 -4  *
2005-06 -73 -1
2006-07 185 3
2007-08 147 2
2008-09 351 4
2009-10 4  *
2010-11 -540 -7
2011-12 135 2

Modelling potential trends in the female prison population

84. Women currently constitute 6 per cent of the prison population in Scotland and have shown a much sharper rate of growth than the global population, practically doubling over the past 10 years.

85. The accelerated rate of increase has been due to several factors

  • An overall population increase as a result of changes to the criminal justice system since the early 2000s, primarily aimed at improving the efficiency of the criminal court system, along with a general upward drift in severity of sentencing.
  • An increased focus in general on the types of crimes women are more likely to commit (notably drugs-related offences, and to a lesser extent, common assault and breach of the peace), combined with consistently high levels of crimes of dishonesty for women at a time when this type of crime has been declining sharply among other groups. The increase in prison receptions for crimes of violence has also been much more marked for women (see Annex A in Scottish prison population projections: 2010-11 to 2019-20).

86. Trends in the female prison population are not modelled separately as part of the main prison population projections as the numbers are too small to allow robust projections over the longer term using the same methodology. In order to provide indicative figures for the purposes of planning, the base female population has therefore been modelled directly using linear regression. Due to the differences in methodology, readers should note that the figures presented here are not commensurate with the main projections shown in Section 4, and should therefore not be used together.

87. There have been several discontinuities in trend for the female population over the past 20 years, and therefore the choice of regression period will affect the robustness of the estimates in the longer term.

Modelling trends for female prison population: 2012-13 to 2020-21 Table C.4

  2011-12 2012-13 2013-14 2014-15 2015-16 2016-17 2017-18 2018-19 2019-20 2020-21
Total average daily population 8,178 8,300 8,400 8,600 8,700 8,900 9,000 9,200 9,300 9,500
Estimated maximum 8,461 8,600 8,700 8,900 9,000 9,200 9,400 9,500 9,700 9,800
1990 based model
Female sentenced 360 350 360 370 380 400 410 420 430 440
Female long term 115 110 110 120 120 130 130 130 140 140
Female short term 245 240 240 250 260 270 280 290 290 300
Female remand 108 100 110 110 120 120 120 130 130 130
Average female population 468 450 470 480 500 510 530 550 560 580
Estimated maximum 504 500 520 530 550 570 590 600 620 640
2000 based model
Female sentenced 360 380 390 410 430 440 460 470 490 510
Female long term 115 120 130 130 140 140 150 150 160 160
Female short term 245 260 270 280 290 300 310 320 330 350
Female remand 108 110 120 120 130 130 140 140 150 150
Average female population 468 490 510 530 550 570 600 620 640 660
Estimated maximum 504 540 560 590 610 640 660 680 710 730

Source: Scottish Government Justice Analytical Services

Notes:

1. It is assumed that the base projections for the total population remain unchanged under the different scenarios, which model the effect of different rates of growth for the female population rather than the impact on the population as a whole. Base projections are for December 2011.

2. Projected figures are estimates and are rounded to nearest 100 for the overall figure, and to the nearest 10 for the female population. Figures for 2011-12 are actual.

3. Life prisoners and recalls are included in the long term population.

4. The breakdown by custody type and sentence length is based on the current female population profile. The estimated maximum is calculated using the average difference over a five year period between the annual daily average and maximum population over the year.

5. Data are derived from a live information management system and updated and quality assured on an on-going basis. The figures shown here may therefore differ slightly from those published previously.

88. The scenarios shown here illustrate the likely range of the estimated future population depending on whether the trend realised over the next 10 years reflects patterns of change since 1990 or 2000, the latter resulting in a more marked rate of growth. The two scenarios give a range for the estimated increase between 25-40 per cent over a 10 year period, reaching an annual daily average of 580-660 by 2020-21 (Table C.4 and Chart C.2).

Chart C.2 Modelled trends for female prison population: 1990 and 2000 based models

Chart C.2 Modelled trends for female prison population: 1990 and 2000 based models

Contact

Email: JusticeStatistics@scotland.gsi.gov.uk

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