Prison statistics and population projections Scotland: 2011-12

4. Prison population projections: 2011-12 to 2020-21

37. The latest set of prison population projections suggest that the daily prison population in Scotland will increase from an annual average of 8,300 in 2012-13 to 9,500 by 2020-21 (Table 4.1). These projections take into account recent trends in the prison population, including the latest available monthly data for 2011. It should be noted that the projections are based on past trends and do not explicitly model the potential impact of future changes in policy or practice on the prison population.

Prison population projections: December 2010 and December 2011
Table 4.1

	December 2010 (average daily population)	December 2011 (average daily population)
2011-12	8,000	8,178 (actual)
2012-13	8,300	8,300
2013-14	8,600	8,400
2014-15	8,800	8,600
2015-16	9,000	8,700
2016-17	9,100	8,900
2017-18	9,200	9,000
2018-19	9,400	9,200
2019-20	9,500	9,300
2020-21		9,500

Note: Figures have been rounded to the nearest 100 and exclude the home detention curfew population. This currently stands at about 370 (average daily population).

38. While the prison population has been relatively high during 2011, the current projections reflect some degree of flattening out compared to earlier projections. This is in part due to the low population levels observed during 2010 and a general slowing down of the rate of increase since 2009. While the latest projections show slightly lower numbers than the December 2010 projections, they continue to confirm the steady upward trend observed since 2000.

39. The prison environment is designed to accommodate the differing needs of diverse types of prisoners, for instance, in terms of custody type, sentence length and stage, and security level. The prison service also holds young offenders and women, and therefore must provide services which are appropriate to their needs and which may be different in some respects from those required by the predominantly adult male population (see the Scottish Prison Service Strategy framework for the management of women offenders in custody). About 10 per cent of prisoners are young offenders, while 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 ten years.

40. The prison population of young offenders and women has historically followed a different trajectory to that for the adult male population, and to this end, Annex A in Scottish prison population projections: 2010-11 to 2019-20 provides a summary of drivers of change for these population groups. The population of female prisoners is not projected separately as part of the main projections since this is a small volatile population which is difficult to model accurately. Indicative figures based on separate analyses are shown in Annex C.

Methodology

41. Reception data for different sentence categories are projected separately for adults (21 and over) and young offenders (under 21) and then converted into populations. The data used are derived from the Scottish Prison Service PR2 management information system.

42. Some groups of prisoners are treated differently from the direct sentenced group for the projections (details shown in Table C.1 in Annex C):

• Remand receptions are projected as a proportion of direct sentenced receptions during 2011.
• Part of the prison population consists of persons who have been previously released under licence or supervision but subsequently recalled into custody. Recall cases are regularly reviewed by the Parole Board for Scotland to assess suitability for release, and it is difficult to estimate how long such prisoners will remain in custody. Therefore this population is projected as a proportion of the long-term population.
• Receptions of fine defaulters are held constant at 2011 levels. This group spends very short periods in custody (around three days) and so any change in the number of receptions makes very little difference to the resulting projected population.
• The population of other types of prisoners is also very low and held constant at 2011 levels.

43. Projected receptions are converted into populations using an approximation of the time spent in custody, or time served factor, for each sentence band (see Table C.2 in Annex C for more detail on time served factors).

44. Projections are based on past trends and do not explicitly model the potential impact of future changes in policy or practice on the prison population. The projection model was run using reception data to December 2011. The underlying trends assumed for all categories of sentenced receptions are trends in receptions since 1972, but the projected trends are based on a range of time periods in order to model more accurately changes of different magnitudes over different time scales.

Sensitivity of population projections: high and low variants

45. A range of methods are required to compensate for the inherent volatility of the population over time, particularly for the smaller groups. Three time periods are used to take account of short, medium and long term trends: these are most recent 10 years, the most recent 25 years, and 1972 to the present. Two different approaches are used: exponential smoothing models the seasonal variation over the year using monthly data, while linear regression models underlying trends using annual data. This gives six projection variants for each sentence category.

46. The main projection is the combination of the most likely projection variant for each sentence category. The sensitivity of the projection to the choice of variant is shown by the upper and lower limits for the range of plausible projections, based on the highest and lowest plausible variant for each sentence category (Chart 4.1).

Chart 4.1 Prison population projections: 2011-12 to 2020-21

47. At present, ten years is the shortest time period used to establish a trend. This makes the projections less volatile in relation to potential short lived spikes in the figures. However, in periods of rapid change such as the ones observed over the past few years, it is difficult to predict to what extent an initial spike may turn out to be the leading edge of a change in trend, or whether there will be a reversion back to earlier trend patterns, as appears to be the case at present. To illustrate the inherent uncertainty involved in making such projections over the longer term, Table C.3 in Annex C shows the historical accuracy of past projections over the past two decades.

48. In this case, the rapid population increase observed between 2006 and 2009 appears to have tailed off. Although the overall trend is still upward, the rate of increase has slowed down. The choice of main variant for the 2011 projections reflects this in that it is in line with the rate of change over the past ten years and gives somewhat lower average daily population figures than the December 2010 projections (Table 4.2).

Prison population projection variants: December 2010 and December 2011 Table 4.2

	December 2010			December 2011
	High variant	Main variant	Low variant	High variant	Main variant	Low variant
2011-12	8,300	8,000	7,000		8,178 (actual)
2012-13	8,700	8,300	6,800	8,900	8,300	7,600
2013-14	9,100	8,600	6,800	9,300	8,400	7,700
2014-15	9,400	8,800	6,800	9,700	8,600	7,700
2015-16	9,700	9,000	6,800	10,000	8,700	7,800
2016-17	9,900	9,100	6,900	10,300	8,900	7,800
2017-18	10,200	9,200	6,900	10,500	9,000	7,900
2018-19	10,400	9,400	6,900	10,800	9,200	7,900
2019-20	10,700	9,500	7,000	11,000	9,300	8,000
2020-21				11,300	9,500	8,000

Note: Figures have been rounded to the nearest 100 and exclude the home detention curfew population. This currently stands at about 370 (average daily population).

49. It may also be noted that the high variant for the December 2011 projections is somewhat higher than that for December 2010. This is due to several factors. Receptions for sentences between six months and four years have shown sustained increases over the past five years. In addition, receptions for sentences of four years and over have fluctuated substantially over the past four years. The high variant therefore represents the scenario where receptions for medium length sentences continue to increase as they have done over the past five years, and receptions for long-term sentences do not stabilise at lower levels.

50. Charts 4.2 and 4.3 provide more detail on the main drivers of the population and potential future trends (the scale on both graphs is the same in order to make them easier to compare in terms of magnitude of effect). It can be seen from Chart 4.2 that the primary driver in terms of volume are adult determinate sentence prisoners (including sentenced for fine default, but excluding life sentence and recalled prisoners).

Chart 4.2 Prison population projections by custody/sentence type

51. As highlighted in §8, it is the population with medium term sentences from 18 months to under four years, as well as the long-term population to a somewhat lesser extent, which have shown the most marked increases (Chart 4.3). However, it can also be seen that there is substantial fluctuation in the short to medium term for some groups, and short-lived but relatively extreme changes in the population may mask longer term underlying trends.

Chart 4.3 Prison population projections by sentence length