Prison statistics and population projections Scotland: 2011-12

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

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