Census 2021: comparisons with ONS population estimates and projections

Prior to the publication of results from the 2021 census the most reliable indication of the Somerset population was from the Office for National Statistics' (ONS) mid-year population estimates for 2020, published in June 2021. ONS publish mid-year estimates for each year to provide insight into population changes between census years. These estimates are used widely for statistical analysis and service planning.

To give an indication of the reliability of these estimates we can compare the results of the 2021 census with the mid-year estimate for 2020. The Somerset population according to the 2021 census is nearly 8,000 higher than the 2020 mid-year estimate, a difference of 1.37%. 

If we break down the results of the 2021 census and the 2020 mid-year estimates into 10-year age groupings, we can see that it was the 20-29 and 30-39 year old age groups which had populations in significantly higher in the 2021 census than expected from the 2020 mid-year estimates. For the 20-29 age group, the 2021 census result is 6.55% (3,700 persons) higher than the 2020 mid-year estimate. In contrast, the population aged under 10 according to the 2021 census was 3.82% (2,200 persons) lower than the same age group in the 2020 mid-year estimates.

When plotted over time we see that the mid-year estimate for 2020 had shown a slowing in annual growth of the Somerset population compared to previous years' figures. Given that 2020 was a year severely disrupted by the COVID-19 pandemic, it is not surprising that there is a level of uncertainty in the mid-year population estimate. At the time of writing (August 2022)  it is very difficult to disentangle the extent to which the apparently low 2020 estimate may have been an error in the ONS mid-year estimate, or whether it is representative of real changes which occurred in the Somerset population during the height of the pandemic.

Alongside annual mid-year population estimates, the ONS also publish projections for Somerset's future population. The most recent of these projections were based on the mid-year estimates from 2018. These projections are widely used in planning for future demand on services. We can see that the projected population for 2021 - based on the 2018 mid-year estimates - is remarkably accurate when compared with the results of the 2021 census, with a difference in population of only 124 people (0.02%).

The closeness between the 2018-based projection for 2021 and the results of the 2021 census suggest that the 2018-based projections can be treated as a reliable resource when looking further ahead - although it should be remembered that these are projections, not predictions.