SPS Management Information Technical Manual

cellWise is the construction method for the data underlying the Accredited Official Statistics, which chains cell occupancy records into stints in the same prison establishment as described in the Technical Manual.  These are further combined into periods of continuous occupancy in any establishment. To occupancy periods are added the first instances of untried, convicted awaiting sentence and sentenced warrants, providing a unidirectional map of changes in legal status (from untried to CAS to sentenced). This limitation is a result of the end-dates of warrants not being recorded on the PR2 system when they expire.

cellWise data are extracted at maturity – typically three months after the period being examined. This allows system administrators and operators time to check and correct the operational data constantly being entered into the PR2 system.

The “longSnap” data can be used to identify transitions from sentenced to remand statuses, as described above. For the 2024-25 official statistics publication[ii], an analysis of the impact of these retrograde transitions on remand estimates was conducted. More information about that analysis is provided in the following sections.

Coherence of matching

The level of match achieved between the longSnap and cellWise sets may be observed in the commonality of Prisoner Numbers held in common over the same periods (financial years).

As shown in Table 1, there were a few individuals who appear in the longSnap data and a much greater number who appear in the cellWise data but not in the daily snapshots. In both cases, the unmatched individuals are a very small minority of the total number of individuals registered as spending time in custody by the accredited official statistics.

Additional individuals in longSnap may be a result of prisoner marked as being “In Custody” in error, and therefore not being allocated a cell. Additional cellWise individuals may be the opposite error, or individuals who arrived and departed in the same day and so were not present at midnight when the snapshot was taken.

Table 1: Discrepancies in individuals present in the cellWise and longSnap datasets

Another factor in determining coherence between the two sources is the difference in dates from an individual’s arrival and departure in cellWise, and their presence and absence in daily snapshots.

A degree of non-alignment is expected between cellWise and longSnap data in the dates of arrival and departure. For example, a person arriving and being assigned a cell during the day will appear in the cellWise data one day before the post-midnight snapshot that registers their arrival in longSnap. This is not the case for departures, as longSnap correctly counts the day before absence as the departure date.

Validity of revisions

We can examine the types of legal status journey charted by snapWise and compare it with the journey expected by cellWise. In the following cases we accept the patterns of remand and sentenced occupancy provided by snapWise:

• Journey types align: both cellWise and snapWise agree on the journey type starting sentenced (“_S”), remaining remand (“R_”) or transitioning to sentenced (“RS”).
• Retrograde transitions: cellWise provides an “_S” or “RS” journey while snapWise produces “SR…” or “RSR…” journey types.
• Missing information populated: the legal status of prisoners is not determined in cellWise and the only available information is provided by snapWise

In these cases the proportion of the occupancy period[i] spent with each legal status is updated with information from snapWise.

Where the snapWise data contradicts the information held in cellWise, rather than overcoming its known shortcomings, the data are not revised for the purpose of the corrected remand estimate.

Results

The results of the snapWise revision to cellWise estimates of remand populations for the period 2020-21 through 2024-25 is provided in the Accredited Official Statistics report[ii].