9. Source Comparison

The cellWise data on which this publication is based is substantially different from other sources of information about the prison population – the preceding statistical publications from Scottish Government (up to 2013-14)[1], the management information snapshots taken throughout the pandemic period published as monthly reports by Scottish Government[2], and the aggregate information produced by the Scottish Prison Service[3].

The major points of difference lie in the average distribution between legal statuses, and the inability in the cellWise data to determine overall sentence lengths.  In spite of this, as shown in Figure 29, the estimate of the overall prison population are closely aligned across the three annual sources:

• across the 5 years where the previous Official Statistics overlap (2010-11 through 2015-16), the cellWise estimate of average daily population is slightly lower by between 0.4% and 0.6%
• across the 9 years where the SPS aggregation overlaps (2014-15 to 2022-23), the cellWise estimate of average daily population is within ± 0.05%

This provides us with a very high level of confidence in using the cellWise data to describe the occupancy and characteristics of people in Scottish prisons.

Figure 29: Comparison of sources for the overall prison population estimation show close alignment

Further disaggregated population group changes are provided in Supplementary Table A2.

9.1 Legal status discrepancies

Due to the lack of consistent information retained on PR2 about the end point of periods spent in prison serving a sentence, prisoners who return to the remand population immediately after a sentence is served continue to be counted as sentenced for the purpose of their on-going Occupancy Period[4].  This aspect of the data construction is explained in further detail in the accompanying technical manual[5].

Based on the snapshots collected to monitor the  prison population from the beginning of the pandemic, there were 656 such retrograde transitions (from sentenced to remand) in the course of 2020-21, growing to 936 in 2021-22 and 1,274 in 2022-23.  In addition there were 125 transitions from CAS to Untried in 2020-21, growing to 168 in 2021-22 and 224 in 2022-23.  No such transitions can be detected in the cellWise data construction.

This leads to the following effects when comparing the cellWise and SPS aggregate estimates of legal status populations over the 6 intersecting years:

• A growing underestimate of the average daily Untried population from 13% (214) in 2020-21, 16% (299) in 2021-22 and 17% (319) in 2022-23
• A shifting variation in the average daily Convicted Awaiting Sentence population, which goes from over-estimating by 61 (32%) in 2020-21 and 58 (25%) in 2021-22 to a slight underestimate of 17 (5%) in 2022-23
• Over-estimate of the average daily Sentenced population by 50-200, or around 4%

These differences over time are illustrated in Figure 30:

Figure 30: Comparison of sources of population estimates by legal status

9.2 Index offences

The offences for which people are imprisoned listed throughout this report are index offences[6].  People may be imprisoned for multiple offences in a single occupancy period.  In such cases we identify the most “serious” offence by the only available metric – the length of associated sentence.  Where no sentences have been passed, the index offence is the offence category with the longest associated sentence on average based on all sentencing warrants over the past three years of SPS warrant data.

9.3 Sentence length ambiguity

Throughout this report the sentence length of prisoners is their “index” sentence length[7].  There is currently no conflicting published source on the sentence distribution of prisoners.  However, live information drawn from the PR2 system can provide the overall sentence length for each individual person in prison when the snapshot is taken.  CellWise cannot use this information, which is over-written on PR2 when it is amended or updated.

Information about sentence length drawn from the cellWise data will therefore always explicitly reference Index Sentence length – the length of the longest determinate sentence applied in the course of an Occupancy Period. 

This information cannot be used to divide the population into “long-term” and “short-term prisoners” (those serving an overall sentence of less than 4 years).  Live (or daily aggregated) information is needed for that population segmentation.

A comparison of index sentence length and overall sentence length is provided in the Technical Manual for 2013-14, the latest year when the previous Official Statistics publication and cellWise data were both available[8].