Long-term fertility assumption used in the National Population Projections, UK

Dyddiad y datganiad:
3 September 2026

FOI reference: FOI-2026-3681

You asked

I am seeking recorded information held by the ONS relating to the long-term fertility assumption used in the National Population Projections, covering the 2018-based, 2020-based, 2021-based and 2022-based projections, together with any subsequent revision published to date.

  1. Any analysis, briefing, methodological note or internal paper setting out how the long-term fertility assumption for the United Kingdom was derived for each of those projection rounds, including the evidence relied upon.

  2. Minutes, notes or papers of the expert advisory panel or equivalent advisory group convened in connection with the fertility assumptions for those projections.

  3. Any recorded disagreement, dissenting view, minority position or challenge — whether from within the ONS or from an external adviser — regarding the level at which the long-term fertility assumption was set, and any recorded response to it.

  4. Any analysis held by the ONS which examines whether an advanced economy with sustained below-replacement fertility has subsequently returned to and maintained a higher long-term rate, and any assessment of the evidential basis for assuming such a recovery.

  5. Any correspondence between the ONS and HM Treasury, the Department of Health and Social Care, or the Office for Budget Responsibility regarding the fertility assumption or the accuracy of previous fertility projections, limited to the period 1 January 2018 to the date of this request.

  6. Any internal review, lessons-learned exercise or assessment of the accuracy of fertility assumptions in previous projection rounds against subsequent outturn.

We said

Thank you for your request.  

Information related to the fertility assumptions for each release of the national population projections (NPPs) you have requested is already publicly available on our website. For each of the releases you have asked about, we provide an explanation of how the assumptions were set and what considerations were factored into their development.  

On our website, you can find the:

Each release includes links to the underlying data and supporting documentation. 

The following documents outline the methodology used for setting the fertility assumptions for each round of projections. These include a summary of input from the Fertility Expert Advisory Panel, including the range of factors which were incorporated into the assumptions. Please note that the 2020-based fertility assumptions were also used for the 2021-based interim set of projections. 

Alongside each release of the NPPs, we have published information on the methodology and processes in use for the development of the assumptions and projections. Please see the following links for the methodology and analysis information provided for each projection round from the 2018-based to 2024-based releases: 

Quality and methodology information is updated for each NPP release (2018, 2020, 2022 and 2024-based). The latest version of this is the National population projections quality and methods guide. 

Following the completion of Census 2021 and the ability to compare the projections to estimates benefitting from census data, we completed a Comparing national population projections to estimates report, which included a specific section on fertility in the form of both the total fertility rate (TFR) and the number of births. This also includes a comparison of the UK constituent countries.  

Due to the size and scope of your request, we are unable to provide a full response to all of information you have requested, as this would exceed the cost limit for FOIA compliance. To further explain this, development of the assumptions for the NPPs takes at least six months and the full extent of the request covers the end-to-end production of NPPs for three rounds of projections where the end-to-end production of NPPs can take up to 12 months. As such, the search would encompass: 

  • staff in three different ONS divisions and in more than three teams exceeding 30 individuals

  • searching in at least four different team Outlook inboxes in addition to inboxes for the 30 individuals 

  • over eight server file folders (for example, there are almost 7,000 files in one folder used for the development of 2018-based NPPs) and up to six SharePoint folders. 

These searches would include the mailboxes of several staff members working at the time across different teams, many of whom will have moved roles, as well as current and archived electronic filings systems.

The search terms we would need to use would also return a number of documents that would need to be sifted to determine whether they are in scope, including for the range of users whom you ask about. Once located, this information would then need to be extracted and collated with any duplicates removed ready for review to determine what could be released. 

The cost limit for FOIA compliance is 24 working hours to determine, locate, extract and collate requested information, and in our view we would need to exceed this to action this request.  

We may be able to provide you with a response if the scope of your request is narrowed down. You may wish, for example, to reduce your request and ask for us to provide the analysis, briefing, methodological note or internal papers created for the most recent projections round or a specific past release. 

Since the release of the 2020-based NPPs we have made Expert Advisory Panel meeting minutes available on request via pop.info@ons.gov.uk.

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