On 28 May 2020, the Welsh Government’s Chief Statistician withdrew the 2018-based local authority population projections and the 2018-based local authority household projections from the Welsh Government’s website.
This was following an announcement by the Office for National Statistics (ONS) that they had found an error affecting the 2018-based national population projections.
National population projections error
The error in the national population projections was caused by incorrect processing of cross-border flows between Wales and England, resulting in the projected mid-2028 population for Wales being approximately 65,000 too low, and that for England being approximately 65,000 too high.
The ONS published corrected projections for Wales on Thursday 11 June, followed by the summary tables on 17 June and the interactive population pyramids on 23 June.
Revised local authority projections
The Welsh Government has now pre-announced the publication of corrected 2018-based local authority population and household projections for Tuesday 4 August. These projections, however, will be calculated in a different way to the projections originally published in February this year.
Change in methodology
As the Chief Statistician stated in his update about the projections earlier this year, two main changes had been made to the method of calculating the subnational projections.
Constraining the projections
The first change was in relation to constraining the local authority population projections to the national population projections.
When we first calculated the local authority population projections using the same methodology as for previous projections, we found that they showed a different long-term trend to the national population projections. While the original 2018-based national population projections were projecting population decrease, the local authority population projections were projecting population increase. Therefore, the decision was made at the time to constrain the local authority population projections to the national population projections. This meant that the sum of the local authority population projections matched the national population projections.
With our technical advisory group (the Wales Subnational Projections group, WaSP), we have decided to revert to the original unconstrained projections.
The decision to constrain the local authority projections had been made due to the error in the national projections. Reverting to calculating the local authority population projections without constraining to the national population projections has shown that they are now more in line with the national trend.
The corrected 2018-based national population projections now show continued projected population increase for Wales over the first 10 years of the projection period, as well as for the full 25-year projection period. This is now broadly in line with the original local authority population projections.
The local authority population projections in Wales have always been based on the principle of being driven by local trends in births, deaths and migration so that they are of most use for planning at the local level. With this in mind, and that the corrected national population projections are now showing a different long-term trend, we have decided to remove the constraining from the local authority population projections. Doing this will also ensure better continuity with previous local authority population projections, and will allow us to follow similar methodology for calculating the national park population projections.
The second change to the methodology referred to in the Chief Statistician’s update regarding migration, however, is being retained. This means that internal migration will continue to be dependent on the size and the age structure of the population of the local authority and the rest of the UK, rather than set at fixed numbers for the duration of the projection period.
There are also no changes to the methodology used to calculate the household projections.
Corrected local authority projections will be published on Tuesday 4 August.
We will then return our attention to calculating and publishing national park population and household projections. We will also continue to explore additional migration variants.
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Head of Population, Census and Welsh Language Statistics