Prison headroom, England and Wales — Walkforward Research

The claim, as registered

Headroom in the England and Wales prison estate, as published in the Ministry of Justice weekly prison population bulletin for Monday 28 Sep 2026, will be 2,076, with an 80% interval of 1,505 to 2,672.

Every line on this track takes that wording with its own target, forecast and interval. The wording is fixed at registration and is never edited.

The founding set

Target	Forecast	80% interval	Naive	Seasonal	Official	Mark expected	Outcome
28 Sep 2026	2,076	1,505 to 2,672	2,094	2,025	—	2 Oct 2026	awaiting bulletin
26 Oct 2026	3,170	2,414 to 3,869	2,094	1,927	—	30 Oct 2026	awaiting bulletin
30 Nov 2026	3,509	2,655 to 4,392	2,094	2,069	1,499	4 Dec 2026	awaiting bulletin
28 Dec 2026	4,226	3,280 to 5,255	2,094	2,728	—	8 Jan 2027Bank holiday, substitute Boxing Day. Resolves against the next bulletin by rule 2.	awaiting bulletin
25 Jan 2027	4,210	3,197 to 5,499	2,094	2,548	—	29 Jan 2027	awaiting bulletin
1 Mar 2027	3,957	2,936 to 5,210	2,094	2,165	—	5 Mar 2027	awaiting bulletin
29 Mar 2027	4,109	3,032 to 5,445	2,094	2,289	—	9 Apr 2027Easter Monday. Resolves against the next bulletin by rule 2.	awaiting bulletin
31 May 2027	4,048	2,864 to 5,456	2,094	2,515	2,099	11 Jun 2027Spring bank holiday. Resolves against the next bulletin by rule 2.	awaiting bulletin

Marks happen when a target resolves, not weekly. Bulletins publish on Fridays for the preceding Monday. From Friday 11 September 2026 a new forecast is posted every bulletin week at 4, 13 and 26 weeks ahead, so the track gains three lines a week. The eight lines below are the founding set, not the cadence.

Model versions v1 and v2 are retained in the register file and scored alongside v3. They are not shown here and are never deleted from the file.

Benchmarks

Naive

The 24 August 2026 headroom, 2,094, carried forwardEdition: None; last published figure

Seasonal naive

2,094 plus the mean seasonal change to the target week from the 2011 to 2026 weekly profileEdition: outputs/seasonal_profile.csv

Official path

Adult-estate supply minus central demand from Figure 4, plus the YCS headroom at the base date (99). Published only where the Statement gives a dated point.Edition: Annual Statement on Prison Capacity 2025, 29 January 2026

Scoring rule

Field. Total-estate Useable Operational Capacity minus total-estate Population in the MoJ weekly bulletin, equal to the bulletin's own Headroom row. The figure as first published is used; later MoJ revisions are logged but do not change the score.
Resolving bulletin. The bulletin whose reference date equals the target date. If no bulletin exists for that date, the first bulletin after it resolves the line, and that bulletin's date is recorded as the resolving date.
Error equals forecast minus outcome, in people, sign kept.
Interval hit means the outcome falls within the 80% interval, inclusive.
Closer means a smaller absolute error than the named baseline, per line. Across the track, the model is said to beat a baseline only when its running mean absolute error is lower over at least 13 resolved lines.
Policy events between bulletins have no effect. A release tranche, a recall change, a capacity measure or anything else occurring between bulletins does not alter the forecast or the score. Events are logged for context only.
Definitional change. If the MoJ changes the meaning of useable operational capacity, the line is still scored against the published figure and the change is logged. The target is the published number.

Provenance

Source of truth

The register file, not this page. The register file is the source of truth. If this ledger and that file disagree, the file wins.

walkforwardresearch/prisoncap · outputs/forecast_register.csv
commit 8df8a8f
OSF registration w9vgr

Pre-registration and data

preregistration/WFR_preregistration_prison_headroom.md

MoJ weekly prison population bulletin, gov.uk, Prison population: weekly estate figures
Annual Statement on Prison Capacity 2025, published 29 January 2026
Seasonal profile, 2011 to 2026, outputs/seasonal_profile.csv