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)[i], the management information snapshots taken throughout the pandemic period published as monthly reports by Scottish Government[ii], and the aggregate information produced by the Scottish Prison Service[iii].

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 35, 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 (2009-10 through 2013-14), the cellWise estimate of average daily population is slightly lower by between 0.40% and 0.55%
• across the 11 years where the SPS aggregation overlaps (2014-15 to 2024-25), the cellWise estimate of average daily population is higher by <= 0.07% averaging 0.02% over this period.

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 35: Comparison of sources for the overall prison population estimation show close alignment 2004-05 to 2024-25

Comparing SPS aggregated population estimates with those produced by cellWise – the data source for this publication, 2004-05 to 2024-25

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

Index offences

The offences for which people are imprisoned listed throughout this report are index offences[iv]. 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.

Sentence length ambiguity

Throughout this report the sentence length of prisoners is their “index” sentence length[v]. 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 directly; it 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[vi].