7 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), the management information snapshots taken throughout the pandemic period published as monthly reports by Scottish Government, and the aggregate information produced by the Scottish Prison Service.
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 demonstrated in Figure 28, 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, the cellwise estimate is slightly lower by between 0.4 and 0.6%
- across the 6 years where the SPS aggregation overlaps (at the time of writing), the cellwise estimate is within ± 0.01%
This provides us a very high level of confidence in using the cellwise data to describe the occupancy and characteristics of people in Scottish prisons.
Further disaggregated population group changes are provided in Supplementary Table A2.
7.1 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. This aspect of the data construction is explained in further detail in the accompanying technical manual.
Based on the snapshots collected as part of the prison population monitoring project initiated at the start of the pandemic, there were 661 such retrograde transitions (from sentenced to remand) in the course of 2020-21. In addition there were a further 128 transitions from CAS to Untried in the same period. All such transitions are not 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:
- Underestimate of the average daily Untried population by 150-200, or approximately 15%
- Over-estimate of the average daily Convicted Awaiting Sentence population by 50-80, or 18-30%
- Over-estimate of the average daily Sentenced population by 40-90, or around 1%
These differences over time are provided in Figure 29:
7.2 Sentence length ambiguity
Throughout this report, where mentioned, the sentence length of prisoners is their Index sentence length. 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 – that 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 required for this 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.
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